Field experiments conducted out to evaluate the performance of a modified thresher machine (El-shams thresher model) for chopping two types of crop residues namely rice straw and cotton stalks. The effect of  two types of cutting knives (swinging and rotating knives), four cutting rotor speeds of 19.63, 23.56, 27.49 and 31.4m/s, and three stalk moisture contents of 10.45, 14.73 and 18.81%, d.b. for rice straw and 8.37, 11.27 and 15.65%, d.b. for cotton stalks on cutting length, cutting efficiency, productivity and total machine losses were taken the considerations. And also, an estimation of power, energy requirements, operating cost and criterion function cost were made. Results indicated that, cutting length <3cm was increased with increasing cutting rotor speed and decreasing stalks moisture content. The maximum value of machine cutting efficiency and machine productivity were 95.8, 94.2% and 1.15, 1.13ton/h for rice straw and cotton stalks, respectively. However the lowest value of energy requirement were 24.63 and 16.26kW.h/ton for rice straw and cotton stalks. Also, minimum values of criterion function cost was 74.68LE/ton and61.67LE/ton  for cutting rice straw and cutting cotton stalks. The optimum operation conditions for cutting rice straw was at cutting rotor speed of 19.6m/s, stalks moisture content of 10.45%, and using swinging knives and for cutting cotton stalks was at cutting rotor speed of  27.5m/s, stalks moisture content of 8.37%, and using rotating knives.