The present study was to test if different ratios of fish meal to soy cake as protein supplements in the diet are not significantly different in effects on feed intakes and performance of growing beef cattle fed urea-treated rice straw (URTRS) as basal diet under local conditions in North Vietnam. Thirty-six male cattle of Laisind (50% local Yellow cattle and 50% Sindhi, both Bos indicus) at around 12 months of age with an average live weight of 124 ± 12 kg at start were used in six groups of the experiment. Six diets named A, B, C, D, E, and F used in the experiment were similar in crude protein (CP) level, except for diet A with no supplementation, but different in fish meal/soy cake ratio based on CP content as follows: diet A 0/0, diet B 100/0, diet C 75/25, diet D 50/50, diet E 25/75, and diet F 0/100. Following a 4-week adaptation period, a comparison period lasted 24 weeks. The total intakes of dry matter (DM), DM from roughage, metabolizable energy (ME), and CP were 6, 7, 6, and 20% higher in diets B-F than diet A with P values ≤ 0.05, 0.05, 0.05, and 0.001, respectively. The average daily gain (ADG) was 50% higher (P<0.001) in diets B-F than that in diet A. Compared to diet A, diets B-F had lower feed conversion ratio (FCR)DM, FCRME, and FCRCP by 30, 20, and 30%, respectively (all at P<0.001). There were no significant differences between diets B-F in the intakes, FCRs, and ADG. The apparent digestibility of CP was 12 to 24% higher (P<0.001) in diets B-F than that in diet A. There was a tendency that the apparent digestibilities of DM, CP, and CF were increasing with increasing level of soy cake in this study. The ratio of fish meal to soy cake did not significantly affect feed intakes, ADG, FCR, and apparent nutrient digestibilities of the cattle. Soy cake can therefore be used to partially or fully replace fish meal as a protein supplement in the diet of growing beef cattle fed URTRS without any negative effects on animal performance.