The purpose of the present study was to investigate the effects of oil diluted by post-injected fuel for CDPF regeneration on engine wear and to find out the characteristic variation of diluted oil according to operating conditions. Experimental studies were made on a 2700 cc, 5 cylinder engine with an after-treatment system. Fuel content in oil increased according to the increase in the duration of post injection. A fuel dilution chart was made to predict the existing fuel content in used oil. The oil analysis method using this chart was validated through the comparison of the results analyzed by GC. The oil contamination by the post-injected fuel caused the quantity of blow-by gas and engine wear to increase and main gallery pressure to decrease.