Abstract: A pot experiment was conducted at National Agricultural Research Center (NARC), Islamabad to study the effect of potash on boll weight, boll size and seed cotton yield in the newly developed highly resistant cotton varieties namely CIM443, CIM109 and CIM446 during the year 1999. The results revealed that boll weight was increased significantly with increasing K2O levels and it was maximum at 200 kg K2O ha-1. Boll size was significantly increased with the application of potash and maximum boll size was observed in plants where K2O was applied @ 200 kg ha-1. However, no significant difference was observed in boll size of different varieties. Seed cotton yield increased significantly in all varieties with increasing K2O levels and it was maximum in variety CIM443 at 200 kg K2O ha-1. In conclusion, the boll characteristics are improved and seed cotton yield increased significantly with the application of an appropriate dose of potash in the newly developed highly resistant cotton varieties.