Abstract: OBJECTIVE To evaluate the intra-tablet coating uniformity and the critical process parameters in an active coating process using response surface method, optimizing the process parameters of the active coating. METHODS Using pioglitazone hydrochloride as a model active substance, active coating on home-made metformin hydrochloride osmotic pump tablets using high efficiency coating machine. RESULTS In the range studied, the effect of pan speed, spray rate, and atomizing pressure on content uniformity decreased in turn. Coating with pan speed at 12 r·min^-1, spray rate at 12-14 g·min^-1, atomizing pressure at 65-75 kPa, the content uniformity of final product met the requirement of chinese pharmacopoeia. CONCLUSION Parameters optimized by the model is suitable for this product, the content uniformity meets the requirement of Chinese Pharmacopoeia 2015 edition.