An attempt was made at Kasetsart University Kampansean Campus, to study the nutritional values of mulberry leaves and their effects on larval growth and cocoon quality. The experiments comprising 2 local (Khun Pi, Mon Noi) and 8 improved mulberry varieties (BR.1, BR.4, BR.5, BR.7, BR.10, BR36, BR.60 and NM.60) were conducted by bioassay on three races of silkworm (Nang Noi, K(,1) XK(,8) and NNxUB(,1) ) in two rearing seasons. The results in the rainy rearing season indicated that the BR.1 and BR.4 varieties were the most favorable for rearing full grown silkworm, Nang Noi race, and the NM.60 variety was the most favorable for K(,1) XK(,8) and NNxUB(,1) races. The progressively increased the single cocoon weight and the shell weight. However, feeding on the BR.36 leaves resulted in poor larval growth and the economic traits of cocoon. The results in the cool rearing season showed that the Mon Noi variety gave the highest silkworm larval growth and the best cocoon characters of the three races of silkworm, followed by the BR.5 variety. As for the BR.36, it still gave the lowest results on silkworm rearing. The nutritional values of mulberry leaves were influenced by the mulberry varieties, seasons and types of mulberry leaves. The results indicated that crude protein, crude fiber and leaf moisture content were higher in the rainy than the cool season but carbohydrate, ash and crude fat content were higher in the cool season in some varieties. However, the larval growth and cocoon quality were somehow related to the crude protein, crude fat, crude fiber and moisture content of leaves of some varieties.