Grains of legumes are affected considerably due to mechanical handling. Traditional threshing practices and operations are laborious and expensive. Timeliness is another important factor. Among legumes, pigeonpea (Cajanus cajan) has considerable potential for adoption by farmers provided some of the major operations such as threshing are mechanized. The present study was aimed at optimizing important operational parameters of a modified spike tooth type thresher and crop parameters influencing threshing of pigeonpea. The thresher consisted of a 562 mm long threshing cylinder of 362 mm diameter, two aspirators, a sieve and screen. Effect of four levels of cylinder peripheral speed (16.4, 18.2, 20.1 and 21.9 m/s) and three levels of crop feed rate (700, 850 and 1000 kg/h) was studied on grain breakage, cleaning and threshing efficiency. Cylinder speed of 18.2 m/s and feed rate of 1000 kg/h resulted in optimum threshing efficiency, cleaning efficiency, breakage and non-collectable losses of 99.56 per cent, 98.63 per cent, 0.68 per cent and 0.27 per cent, respectively.