Seven evaluation parameters were studied in the present work to evaluate chopping operation of cotton stalks (variety Giza 86). Experiments were carried out at the experimental farm, Sakha Agric. Res. Station, Kafr EL-Sheikh Governorate . In general rotary mowers functioned satisfactorily in cotton stalk chopping. The resuts revealed that the highest value of actual field capacity was 1.50 fed /h. at forward speed 5.0 km/h and P.T.O. rotating speed of 540 r.p.m . At these speeds the machine had the lowest cutting energy of 16.5 kw.h/fed. Also, the highest value of cutting efficiency of 95.0 %, chopping efficiency of 94.9 % and degree of destruction of the pink bollworm of 83.3 % were obtained at forward speed of 1.6 knVh and P.T.O. rotating speed of 540 r.p.m. The lowest cored and variable costs of 8.35 LE/fed were estimated at these weeds .