This study was carried out to determine optimum protein requirement in growing slender catfish (Silurus microdorsalis). Growing slender catfish averaging 35.0 ± 0.01g (mean ± SD) were fed one of six experimental diets containing 35, 40, 45, 50, 55 and 60% crude protein (CP) with isocaloric gross energy of 17.7 kJ/g for 8 weeks. White fish meal is major protein source in the experimental diet. After 8 weeks of feeding trial, Average weight gain (WG) of fish fed 45% CP diets were significantly higher than those of fish fed 30, 50, 55 and 60% CP diets (P < 0.05), however there were no significant differences between fish fed 40 and 45% CP diets. Average feed efficiency (FE) of fish fed 45% CP diets were significantly higher than those of fish fed 30, 55 and 60% CP diets (P < 0.05), meanwhile there was no significant differences in FE among fish fed 40, 45 and 50% CP diets. Average specific growth rate (SGR) of fish fed 45% CP diet was significantly higher than those of fish fed 30, 55 and 60% CP diets (P < 0.05), but no significant differences were observed in SGR between fish fed 40, 45 and 50% CP diets. Average protein efficiency ratio (PER) of fish fed 35% CP diets were significantly higher than those of fish fed 30, 50, 55 and 60% CP diets (P < 0.05), however there were no significant differences between fish fed 40 and 45% CP diets. Based on the one-way ANOVA test, the broken-line regression analysis, second order polynomial analysis on weight gain, the optimum dietary protein requirement level could be greater than 44.4%, but less than 47.4% in growing slender catfish.