Cotton (Gossypium hirsutum L.) is considered as the most valuable cash crop of Pakistan. During last decade, its yield has been declined due to various biotic and abiotic factors. Among abiotic factors, improper use of fertilizers is considered very important specially regarding plant defense and yield. This study was conducted to evaluate the effect of different levels (0, 40, 80 and 120 kg ha-1) of K fertilizer (K2O) on different growth parameters of two commercial Bt cotton cultivars (CYTO-301 and IUB-2013) and one non-Bt cultivar (CYTO-142) during 2016 and 2017. Maximum plant height (124-134 cm), dry matter contents (915-1005%), fruiting point (441-462), bolls per plant (96-139), average boll weight (4.2-5.2 g) and seed cotton yield (2524-3175 kg ha-1) and minimum shedding (43-73%) were observed in plots receiving highest dose of K (120 kg ha-1). The CYTO-103 cultivar was found more responsive to K fertilizer as compared to rest of cultivars (CYTO-142 and IUB-2013). Concluding, ideal dose of fertilizer is very important (120 kg ha-1 in our case) for optimum growth and production of good quality fiber with enhanced seed cotton yield.