This study aims to determine the optimum fattening period according to various fattening systems under controlled conditions for the first time in lamb fattening in Turkey. The material of the study consists of 39 Tuj and 39 Hemsin genotype male weaned lambs that are 2.5-3 months old. The Tuj and Hemsin lambs in the study have been separated into 3 equal groups through the systematic sampling method and subject to 90-day fattening. The amount of feed in terms of consumed dry matter for 1 kg of increase in live weight of lambs in the intensive system was calculated as 5.69 kg and 5.90 respectively for Tuj and Hemsin lambs. The optimum fattening period in the intensive system for Tuj and Hemsin lambs occurred between the 70th and 84th days. The marginal cost-marginal income equality was not achieved throughout the fattening in the extensive and semi-intensive fattening periods for both genotypes. Thus, it is estimated that the optimum fattening period in the extensive and semi-intensive systems is over 90 days. The fattening period with the highest marginal income was on the 70th, 84th, and 42nd days respectively for the extensive, semi-intensive, and intensive systems for Tuj lambs and this was on the 70th, 28th, and 56th days respectively for Hemsin lambs. In conclusion, it was observed that the optimum fattening period in Tuj and Hemsin lambs was earlier in the intensive fattening period compared to other fattening periods.