Abstract: This paper introduced the flattening test method for polyethylene (PE) pipe fittings and presented a comparative investigation on the experimental results and variation trends between the flattening test and hydrostatic strength test at 165 h and 80 ℃ through exact tests for the weldline strength of pipe fittings. The results indicated that there was no crack found in the flattening test of weld line, while the specimen was confirmed to be qualified in the hydrostatic strength test. However, when the specimen was determined as a qualified product in the hydrostatic strength test, there are two cases for the flattening test, i.e. with cracks and without crack. The flattening test was found to detect the weldline strength of PE pipe fittings quickly and effectively, and it could meet the requirement for injection molding so as to improve the product quality by adjusting the processing technology and modifying the mold schemes.