The objective of this study was to understand the effects of inbreeding depression in established lines of Taiwanese red tilapia (Oreochromis mossambicus × O. niloticus), by comparing of body weight (BW), standard length (SL), total length (TL) and accumulated mortality (AM) data for control and 6-generation full-sib inbreeding groups using t-test analysis. In the control group, the average BW, SL, and TL values of three-month-old fish of F3 were significantly larger than those of F1 and F2 (p<0.05). There were no significant differences in AM values among F1 to F6 (p>0.05). In the inbreeding group, there were no significant differences in the BW, SL, TL and AM values of three-month-old fish between F1 and F2 (p>0.05), but after F3, the average BW, SL and TL decreased with later generations from F3 to F6 (p<0.05), and AM increased with later generations (p<0.05). This suggests that the traits of BW, SL, TL and AM were significantly affected by the level of inbreeding, especially for SL and AM. The mortality due to inbreeding depression occurred usually at the ages of one month old and two months old.