The auther has carried out various studies on low cycle fatigue by cantilever rotatory bending tests on circular notched bars¹³⁾. In this paper, he has conducted some plastic fatigue tests of flat specimens of mild steel plates by using a pulsating tention testing machine, and confirmed the great influences of cycling rate and load cycle patterns, in the low cycle region under about 10³ cycles, where the phenomenon of sectional contraction due to dynamic dreep dominates. Disregarding this creep effects, it is of no use to discuss the flattness or the slope of S-N diagrams. For the analysis of this fundamental problem, the need of co-operating works under a standardized system must be recognized with attention.