Aiming to obtain the desired cutting lengths from wheat straw, to suit animal feeding and other different uses, proper wheat straw chopping mechanism was developed. That mechanism, involved a conical cutting rotor drum and a coinciding concave, to apply the theory of shearing instead of hammering theory. The straw cutting performance of the developed mechanism was tested, and evaluated under different drum speed, and straw moisture content levels. The evaluation based on the percentages of four cutting length categories, and the uniformity of chopping lengths. The obtained results concluded that the maximum percentages( 84.7 %)  in wheat straw cutting length of less than 30 mm was obtained at cutting speed of1400 rpm (45.7 m/s), feeding rate of 1 kg/sec and knife clearance of 30/20 mm. While, the minimum percentages (25:30%) of that length categories were accomplished the lower cutting speed and the higher straw moisture content levels, at the same feeding rate and knife clearance levels. In addition, the maximum uniformity value(97.8%) for the chopping straw lengths were obtained at cutting speed of1200 rpm (39.25 m/s) and the same mentioned feeding rate and knife clearance. The results also recommended inserting the conical shape drum and its coinciding concave in the construction of the Turkish threshing machine in order to increase the percentages of acceptable fewer cutting length categories by about 7% , and the length uniformity of chopped wheat straw by about 20%.