Abstract The plastic deformation propagation speeds of steels are determined by using photo-elastic coating and high speed photography method.The results indicate that the plastic deformation propagation speed of metal under cyclic loading is related to not only internal factors of crystal lattice and microstructure etc, but also external factors, such as loading frequency, stress concentration etc. The plastic deformation propagation speed of 1Cr18Ni9Ti stainless steel with face-centred cubic lattice is greatly higher than that of 40Cr steel with body-centred cubic lattice. In essence. critical loading speed effect should be connected with the plastic deformation propagation speed of metal.FENG Zhongxin,associate professor, Xi'an Jiaotong University, Xi'an 710049