In this study, data from Brown Swiss beef cattle (45), grown under feedlot conditions were used to determine the effect of climate change on seasonal performance differences in the West Mediterranean type of climate in Turkey. Initial average weight of cattle with about 6 months old was 197 kg and reached 442 kg final weight at the end of the experiment. Feed-Lot period lasted for ten months. Average daily live weight gains (DLWG) of the animals for the seasons of spring, summer, autumn and winter were 0.965, 0.835, 0.940 and 0.685 kg respectively. The data was subjected to one-way analysis of variance and Tukey test was performed to compare the average DLWG of the animals in different seasons. The results showed that while there were no significant differences (P>0.05) in DLWG of animals in spring, summer and autumn, the performance of animals in winter was significantly (P<0.05) lower than those in spring and autumn. In addition, there were no significant differences (P>0.05) in DLWG of the animals in summer and winter. It can be concluded that under the West Mediterranean conditions, the performance of Brown Swiss cattle were better both in the spring and the autumn seasons than other seasons and well-suited to the feedlot beef systems. The overall performance of cattle in indicated that animals may suffer from heat stress during summer and winter, causing a decrease in performance in the climate conditions of the West Mediterranean region of Turkey.