ABSTRACT Due to competition between humans, industry and livestock for crop usage, there is need to identify alternative plant protein sources readily available and inexpensive. Beniseed (Sesamum indicum L.) cake is considered to replace soybean (Glycine max [L. ] Merr.) meal as plant protein source for animal feed. This study was conducted to evaluate substitution effects of Nigerian white beniseed cake (BSC) for soybean cake (SBM) in diets of broilers, and determine optimum substitution levels. Replacement levels of BSC for SBM in diets were 0%, 25%, 75%, and 100%. A 56-d feeding trial was conducted on 264-d old unsexed Cobb broilers, randomly assigned to four treatment groups of 66 birds each, with three replicates of 22 birds each in a completely randomized design. A quadratic type optimization function was used to determine optimum BSC levels that significantly affected growth, carcass and blood indices. Diets 100% BSC had the least body weight at 28 d, final body weight, average daily feed intake (ADFI), and carcass weight (P < 0.05). Diets with 25%, 75% and 100% BSC had higher efficiency factor, abdominal fats and white blood corpuscles than 0% substitution diet (P < 0.05). Diet 25% BSC showed significantly higher dressing percentage (P < 0.05), and diet 0% BSC had higher serum protein and urea compared to the other diets. The quadratic function indicated that ADFI was significantly optimized at 25% BSC+75% SBM, with R2 = 0.992 and P-value 0.051. The threshold of 25% to 75% substitution levels of BSC can replace SBM in the diets of Cobb broilers without detrimental effects.