A four year field trial was carried out to determine the effect of application rate and form of K fertilizer on mulberry (Morus atropurpurea Roxb.) during 1999~2002 and a silk worm (Bombyx mori Linne) experiment to determine their consequential effect on silk cocoon yield and quality in summer of 2001.The results showed that in comparison with the leaf yield in Treatment Zero K,it increased by 23.9%,31.7% and 36.0% in Treatments K₂O 150,300 and 375 kg hm^-2 a^-1,respec tively,and that when the K application rates were the same,the two different forms of K fertilizer,K₂SO₄ and KCl,were quite similar to each other in effect on mulberry leaf yield.Crop response to K application seemed to increase with the time of the experiment and the post-effect of the application was also quite significant.Leaf nutrient determination showed N,K and S contents increased whereas Ca,Mg and Zn decreased with K application in the form of K₂SO₄,but no change for P content.The results showed that mulberry nutrient uptake increased greatly with K application,which led to better fertilizer utilization efficiency.As a result,the essential amino acids,total amino acid,protein,sugar and fat contents in the leaves increased with the K application rate.KCl also improved leaf quality but not so much as K₂SO₄ did.The silk cocoon quality test showed that silk worms fed with mulberry leaves from K treatments produced cocoons better in quality in terms of silk worm weight,cocoon weight,reelable cocoon rate,filament length,reelability and silk neatness.The higher the K application rate,the better the cocoon quality.And the effect of K₂SO₄ was more significant than that of KCl in this aspect when the K application rates were the same.