Abstract: There were many cracks in the cooling oil tubes wrapping the cyclinder of the fuel oil cooling case for an areo-engine after high and low temperature testing. The fracture reasons were analyzed through macroscopic observation and fracture observation of the cracked oil tubes, hardness test and metallography examination for tube material. The results show that the cracks originated from the stress concentration of scathing grooves parallel to the tube axis on the surface of oil tubes. The fracture of the tubes was toughness cracking. Some water remained in part of the tubes during air tightness test. Residual water turning into ice can produce high expansion stress in low temperature zone during high and low temperature testing; meanwhile the tubes in the low temperature stage can produce high shrinkage stress. The repetitive high and low temperature tests led the deformation, hardening and plastical thinning of the tube. And finally the tube ruptured when the superimposed stress exceeded the strength of the material in the low temperature stage.