Food resource is an important factor for individual survival and reproduction. Diet has been a crucial aspect for understanding wildlife habits, which also provides the baseline information for conservation and management. Due to the rareness and illusiveness of Formosan black bears (Ursus thibetanus formosanus), diet analysis based on their scats are generally limited by spatial and temporal sampling error. Stable isotopes analysis has been used as a powerful tool in diet studies. I used carbon and nitrogen stable isotopes ratio in hairs to investigate the diets of Formosan black bears. A feeding trial study on captive bears was conducted to estimate the turnover rate and hair growth rate, which were used to refer dietary seasonal shift. The half-life time of the hair metabolic pool was 20±9.2(mean ±standard deviation, n=3) days. The growth rate of hairs was 1.45 cm and 0.23 cm per month during May-Nov. and Dec-April, respectively. Three years of wild bear hairs were collected in Yushan National Park: 1998 (n=6), 2000 (n=7), and 2008(n=14). The δ13C values of the hair samples from the 3 years were-22.9±1.3, -22.9±0.6‰ and -22.8±1.2‰; the δ15N values were 4.4±0.7‰, 3.9±0.3‰, and 4.6±1.2‰, respectively. There was no significant difference among years for both δ13Cand δ15N values. However, the δ15N values of hair from 2008 in acorn season were significantly heavier than non-acorn season (Paried-t test; df=13, t=4.466, p=0.001), but no significantly different in the δ13C values (Paried-t test; df=13, t=-0.535, p=6.02). In the 2008 acorn season, the δ13C和δ15N value were -22.9±1.0‰ and 5.4±1.3‰, respectively; on the other hand, the δ13C和δ15N value in the non-acorn season were-22.8±1.3‰ and 4.3±1.2‰ respectively. The δ15N value in the acorn season was higher than that in the non-acorn season (Paired-t test；df=13, t=4.466, p=0.001), but no seasonally significant difference was detected for the δ13C value (Paired-t test; df=13, t=-0.535, p=0.602). The Monte Carlo simulation indicated that vegetation and animal food contributed to 67-90% and 10-33% of bear diets, correspondingly. During the acorn masting season, bears tended to be more carnivorous, which may be related to seasonally increasing abundance of ungulates in the study area.