In order to study the protein sparing effect of l-lysine from weanling pig to market weight, a series of feeding and metabolism trials was conducted. In experiment 1, a total of 192 three way crossbred pigs (Landrace x Large White x Duroc) weighing app. 10 ㎏ initially were used for four weeks to determine the effect of supplemental lysine when added to a low protein diets. In experiment 2, a total of 120 three way crossbred pigs were used for 11 weeks from 28 ㎏ to 90 ㎏. The results obtained are as follows: (1) It was found that the dietary protein level of 18% fed during period of 10 to 23㎏ was adequate for the daily gain and feed efficiency. Pigs fed the dietary protein level of 20% were not different in rate of gain and teed efficiency with pigs fed the dietary protein level of 18%. When pig were fed the dietary protein level of 16% + 0.2% l-lysine, rate of gain was similar to pigs fed the dietary protein level of 18%, but feed efficiency was slightly improved, In economic analysis, feed cost required per ㎏. of body weight gain was less for pigs fed the dietary protein level of 16% + 0.2% l-lysine than other levels. Therefore, it was apparent that weanling pig diets supplemented with 0.2% l-lysine was able to spare 2% of dietary protein. (2) The dietary protein level of 16% was adequate for growing pigs from 28 to 53 ㎏. Rate of gain and feed efficiency of pigs fed the dietary protein level of 14% + 0.2% l-lysine was similar to pigs fed dietary protein level of 16%.There were no difference in digestibility and nitrogen retention of diets supplemented with 0.2% l-lysine with other dietary protein level. Therefore, diets supplemented with 0.2% l-lysine could spare 2% of dietary protein for the growers. (3) The dietary protein level of 14% was adequate for finishing pigs from 53 to 90 ㎏. In the rate of gain, pig fed the dietary protein level of 12% supplemented with 0.2% l-lysine was slightly higher, but improved effect in rate of gain during finishing period was somewhat lower than growing period. However, improvement in feed efficiency during finishing period for l-lysine fed group was more remarkable than in rate of gain. Digestibility and nitrogen retention of diets by adding 0.2% l-lysine were not affected. (4) Although there was no significant difference, dressing percentage and back fat thickness was slightly increased, and loin aye area was slightly decreased when pigs were fed dietary protein level of 14% supplemented with 0.2% l-lysine from 28 to 53 ㎏ and 12% supplemented with 0.2% l-lysine from 53 to 90 ㎏. (5) In economic analysis feed cost required per ㎏ of body weight gain for pig fed the dietary protein level of 14-12% supplemented with 0.2% l-lysine was less than that for those fed the other dietary proten levels. Based on the above mentioned results, it may be concluded that adequate level of dietary protein for weanling-growing-finishing pig would be 18-16-14%. It would also be clear that supplementation of l-lysine at 0.2% level in the pig rations from weanling to market weight might be able to spare 2% of dietary protein.