This study was carried out to develop soybean thresher which is able to reduce the soybean threshing damage in comparison to the conventional thresher. A threshing cylinder with different diameter of 480 and 384 mm at each end and with one quarter disc pegs of 60 mm radius was developed and attached to the prototype thresher. A conventional thresher which has a threshing cylinder with $wedge$ type threshing pegs and same diameter of 480 mm at each end was used for comparative test. A series of comparative performance test was conducted using sun-yu and chung-ja soybean. For sun-yu bean, which is white and usually used for soybean paste and soy sauce, the ratio of damaged beans of prototype ranged 2-3% for 330-360 rpm which is recommended cylinder speed by manufacturer. The ratio of damaged beans of conventional thresher was 3-4% for the same range of cylinder speed. chung-ja beans with black color usually shows high damaged ratio compared with white beans, thus cylinder speed of 250-300 rpm is recommended by manufacturer to reduce the damaged ratio. For this range of cylinder speed, the damaged ratio of prototype was 1.3-1.4% and it was 2.7-6.1% for the conventional thresher. Thus prototype is able to reduce the damaged ratio 1.5-5.0% compared with conventional thresher. Prototype shows 0.4% of unthreshed soybean ratio for sun-yu bean in the optimum range of cylinder speed and it was 0.87% for the conventional thresher. For chung-ja bean, the ratio of unthreshed soybean was almost same for both prototype and conventional thresher with the value of 4.0%. The reason of high unthreshed soybean ratio for chung-ja bean compared sun-yu bean is due to the high seed moisture content of 29.11% which is much higher than that of the recommended.