Effects of different carbon to nitrogen ratios in the biofloc system with sugarcane molasses as organic carbon source on water quality, growth performance, and body composition of common carp (Cyprinus carpioL.) fingerlings were investigated in a 90-day trial. A total of 450 fish (14.17 +/- 0.36 g) were fed twice daily with commercial diet at 5\% of their body weight under a 12:12 light:dark regime and cultured in 1 control (without extra carbon source) and 4 experimental treatments (C/N: 11:1, 15:1, 19:1, and 23:1) with 3 replicates at a density of 1.06 kg m(-3)per tank. Based on the results, the water electrical conductivity (EC) and total dissolved solids (TDS) (measured daily), flocs volume (FV), and total suspended solids (TSS) (measured once a week) were increased from C/N 11:1 to C/N 23:1. The variation patterns of the average levels of total ammonia nitrogen, nitrite, and nitrate (measured once a week) were similar and showed a decreasing trend with an increase in C/N ratio. Total heterotrophic bacteria was also positively affected by increasing C/N to 23:1. The highest fish final weight (59.36 +/- 0.24 g), weight gain (45.21 +/- 0.47 g), and protein efficiency ratio (PER, 1.77 +/- 0.04) were recorded in C/N 19:1. The lowest (1.46 +/- 0.03) and highest (1.51 +/- 0.05) feed conversion ratio (FCR) and the highest (1.60 +/- 0.03\%/day) and lowest (1.49 +/- 0.01\%/day) specific growth rate (SGR) were observed in C/N 19:1 and the control group, respectively (P < 0.05). By increasing the C/N to 23:1, FCR increased and the feed intake, final weight, weight gain, PER, and SGR decreased. The highest crude protein content (77.31 +/- 0.43 \%DW) and the lowest crude lipid content (12.13 +/- 0.32 \%DW) of the fish carcass were observed in C/N 19:1 while these values for flocs were not significantly affected by C/N ratio. Overall, the biofloc system with C/N 19:1 improved the water quality and growth performance of common carp while not negatively affecting the carcass proximate analysis.