This study was carried out to investigate the effects of parity, stage of lactation, supplying nutrients, condition of` management, milking hygiene, calving season and milking season on the milk yield, milk components, pH and bacterial quaky of raw milk samples collected from 56 Holstein cows of 7 ranches in Chju-Do area from July, 1987 to June, 1988. The results obtained from this study are summarized as follows: l. The average daily milk yield, 305 day corrected milk yield and milk fat yield were 20.87㎏, 6285.88㎏ and 236.09㎏ respectively. The milk yield and milk fat yield of 3rd parity were higher than that of other parity group. In the effect of calving season, the performance of Winter calving cows showed the highest rasults and that of Autumn calving cows were the lowest. The performance of milk yield was significantly effected by the level of nutrients, conditions of management and milking hygiene. 2. In the aspect of milk yield and milk fat yield in the lactation stage, the milk yield was highest at the first one month. In the effect of milking season, the results in April and June were the highest, and those in January and February were the lowest. In the effect of milking time, the yield of milk was higher in the morning than in the evening. 3. The average values of raw milk composition were found to be milk fat, 3.75%; protein, 3.2% lactose, 4.57 %; total solids, 12.57 %; solids-not-fat, 8.75 %. The component ratio of milk fat, total solids and solids-not-fat of 3rd parity were higher than that of other parity group. But in the component ratio of protein and lactose, there were no significant differences among all the parity groups. The milk fat percentage of Winter calving cows were the highest and that of Summer calving cows were the lowest, in the effect of calving season. In contrast to the milk yield. higher percentage of milk fat appeared in the morning than in the evening. The percentage of all the kind of milk components were higher in the milk produced from cows supplied sufficient nutrients than that produced from cows supplied deficient nutrients, but the condition of management and milking hygiene had no effect on the milk fat percentage. 4. In the effect of the lactation stage, the milk fat percentage was lowest at 5 months after calving but the percentage of protein and total solids were lowest at the first months. Referring to the monthly variation in average raw milk compositions, the contents of fat and total solids showed the lowest results in July and the highest in January. But protein content were lowest in August and highest in December. 5. The average values of the pH, titratable acidity and number of Somatic cell of raw milk found to be 6.72, 0.173 and 26.2 x 10⁴/ml respectively. These values were considerably affected by the management condition and milking hygiene of the experimental ranches. 6. The average number of living bacteria and coliform bacteria were 13.49 × 10^6 cfu/ml and 175.43× 10³ cfu/ml respectively. In the bacteriological quality of raw milk samples, there were significantly great differences among the experimental ranches.