In order to explore the food composition of mantis shrimp Oratosquilla oratoria, we analyzed carbon and nitrogen stable isotope ratios(δ¹³C and δ¹⁵N) in O. oratoria and their preys in Honghai Bay in May 2015. The results showed that the average δ¹³C and δ¹⁵N values of O. oratoria were -17.1‰±0.5‰ and 12.7‰±0.7‰, in range of -18.1‰--16.3‰ and 10.9‰-13.5‰, respectively. Preys of O. oratoria were fishes, shrimps, bivalves, crabs and copepods. Bivalves were the most important prey of O. oratoria by taking 38.6% on the average of the total food sources, followed by crabs(average 22.9%), copepods(average 16.0%), shrimps(average 13.6%) and fishes(average 8.9%). The average trophic level of O. oratoria was 3.01±0.22, on top of all their preys, and fishes, shrimps, bivalves, crabs as well as copepods' trophic level were 2.98±0.15, 2.89±0.16, 2.00±0.20, 2.78±0.21 and 1.77±0.12 respectively. Statistical analyses revealed significant positive correlation between the body weight and δ¹⁵N value of O. oratoria, indicating that the body size affected the trophic level of O. oratoria.