This study aims to investigate the effects of betaine, added to broiler rations at different levels, on fattening performance, carcass, some internal organs weights, and serum parameters. 192 mixed-sex one-day-old broiler chicks were used during the experiment. The experiment was conducted with four groups in total, including one control and three experimental groups. All the groups were divided into four sub-groups, each consisting of 12 chicks, and the total number of chicks in each group was determined to be 48. No additives were added to the feeds of the control group throughout the research. 0.3 g/kg (Bet-0.3), 0.5 g/kg (Bet-0.5) and 0.8 g/kg (Bet-0.8) betaine (Betamar®) were added to the rations of the experimental groups, respectively. Rations were prepared as isocaloric and isonitrogenic. The animals were given feed and water ad-libitum. In the study, The rations containing crude protein (CP) of 23% and3000 kcal/kg metabolic energy (ME)until the day 1-21 and CP of 20% and3224 kcal/kg ME until the day 22-42 days were given in the treatment groups. At the end of the study, statistically significant differences were observed in the experimental groups in terms of body weight (BW) and body weight gain (BWG) when compared with the control group. The highest BW and the highest BWG were observed in the Bet-0.8 experimental group. No statistically significant difference was found in terms of feed consumption, feed conversion ratio, carcass characteristics, weights of some internal organs, and serum blood parameters (P>0.05). It was concluded that adding 0.8 g/kg betaine to broiler rations increase fattening performance and can be used safely.