To investigate the competition mechanisms between pasture plants and crabgrass in orchardgrass and Ladino clover swards and to find effective protectional method of the pasture plants from the competition between crabgrass, a series of following experiments was carried out during the period from April 1973 to October 1913 at Kangwon National University, Chunchon. Experiment 1. the growth and development of crabgrass in orchardgrass/Ladino clover mixtures with different vegetations Experiment 2. the influence of crabgrass on the growth of pasture plants in orchardgrass and Ladino clover swards Experiment 3. the factors affecting the competition between crabgrass and orchardgrass, and Ladino clover Experiment 4. the effects of cutting frequency and cutting height in summer season on the vegetations in orchardgrass and Ladino clover swards mixed with crabgrass The results are summarized as follows: Experiment 1. 1. The crabgrass seeds buried in 1㎡ of orchardgrass/Ladino clover mixtures numbered 530∼610 under good vegetation, 670, poor vegetation and 790, barren place respectively. 2. A rush grow th of crabgrass was observed where the density of pasture plants w as low or the pasture sward was barren, but dominance ratio of pasture plants did not show any significant effects on the growth and development of crabgrass. 3. Dry matter production of crabgrass recorded 12 g/㎡ in the pasture dominated by orchardgrass, 35 g/㎡ in the pasture dominated by Ladino clover and 1,252 g/㎡ in the barren place. Experiment 2. 1. Crabgrass seedlings in the spring sown pasture 6 weeks after seeding were counted to 70∼120 plants/㎡. 2. The most of orchardgrass was smothered where crabgrass was not removed, but in case of Ladino clover the smothering effect of crabgrass was not apparent compared with the case of orchardgrass. 3. Dry matter yield of last cut of orchardgrass in the plot not-removed crabgrass (0.14 t/㏊) was significantly (P$lt;0.05) lower than that in the plot completely-removed crabgrass (3.90 t/㏊). However, the dry matter yield of Ladino clover in the plot not-removed crabgrass (1.49 t/㏊) did not show any difference with that in the plot completely removed crabgrass (1.73 t/㏊). On the other hand the plot mono-cultured crabgrass produced 0.66 t/㏊. 4. It was found that abundant fertilization of nitrogen in mid summer seemed to be unfavorable for growth of orchardgrass on account of increased competive ability of crabgrass. Experiment 3. 1. In pot culture, root competition was observed clearly but shoot competition was not found apparently. 2. In root competition, pasture plants was depressed by crabgrass in summer but the competition was not occurred in autumn. In dry matter production of crabgrass, the plot not-separated is root competition produced 2.4∼2.6 times (P$lt;0.05) more than the plot separated in root competition in mid summer (high temperature condition), while significant difference was not found in autumn (low temperature condition). In high soil moisture condition, both pasture plants and crabgrass made vigorous growth in summer but only pasture plants continued the vigorous growth in autumn crabgrass gave no response to different soil moisture conditions in autumn. Experiment 4. 1. Data revealed that pasture plants affected harmfully by the cutting treatment which was practiced to control crabgrass in mid summer when pasture plants were depressed. 2. Cutting treatment resulted to control crabgrass beared less meaning in Ladino clover sward. 3. Relatively frequent high cutting, such as 20 days cutting interval and 12㎝ cutting height, was thought to be effective to some extent on control of crabgrass in asture swards. From the results of the series of four experiments, firstly, it is concluded that pasture swards should be maintained to keep high density of pasture plants which prevents the invasion of crabgrass. Secondly, it is also suggested that the stage of seedling establishment shou