Abstract: To establish the potential control of Semiaphis heraclei Takahashi by Harmonia axyridis Pallas, we evaluated predatory function in and control efficacy of H. axyridis, using celery Apium graveolens cv. Ventura as host plant and S. heraclei as prey. When aphid densities were set a 40, 80 and 120, predation by H. axyridis increased significantly with prey density. When prey densities were 160, 200 and 240, numbers of preys predated by the ladybirds were not significantly different between prey density groups. Prey density showed significant impact on predation by the ladybird 4th larvae, which could be fitted with Holling-II model. The instant attack parameter a' of the ladybird 4th larvae was 1.1862, the disposal time was 0.004, and the Na max was 210.9. The search effect of ladybird could be described by S=1+0.00474 N. Predation of the prey by the ladybird 4th larvae fitted well with quadratic regression equation, which showed an increase and then a decrease in predation with the increase of ladybird, when the density of aphids was set at 500. The equation also indicated a Na max of 186.26 and an optimum beneficial/pest ratio of 1:83. The results may be significant for biocontrol of S. heraclei using Harmonias axyridis in organic cultivation of celery.