Grease deterioration of mineral oil type grease was investigated using sealed deep groove ball bearings (6203) and bearing life test rig. The bearing test was performed under 3,000 rpm and 245 N radial load at 120。C. Grease taken ken tested bearings was checked in using certain evaluation methods, for example, oxidation induction time (OIT), FT-IR peak intensity, leakage .ate, bleeding .ate, total acid number. (TAN) and bearing noise increase. Within these test conditions, OIT, TAN, bleeding rate were changed gradually in accordance with bearing operating time and showed useful indices for deterioration of grease.