Abstract: After being processed with cold-drawing and rolling processes，the cylinder-serviced pipe is turned into the case of the hydraulic cylinder. The inner surface defects of the cylinder case as commonly found include porosity， inner fold，pit and straight line mark，etc.，which leads to the failure of the cylinder. Because there are a number of manufacturing procedures for making the cylinder case，it is so difficult to identify the causes for the said inner surface defects just by the macroscopic appearance thereof. Accordingly the said various inner surface defects of the cylinder case have to be analyzed by means of the metallographic microscope，and the scanning electron microscope（SEM）so as to find out the method for tracing the source of the defects，and then based on the causes for the defects as identified，relevant corrective actions are determined.