In this study, five types of load controlled fatigue tests were carried out under variable stress amplitude in order to clarify the stress-strain response and the fatigue strength of SS41 steel plate specimen. That is, those tests were constant stress amplitude test, step test, repeated high-to-low and repeated low-to-high block loading test and block random loading test. In each test, stress-strain loops were recorded by X-Y plotter from the start of cycling to visual crack initiation by using the strain gauges put them on the test piece. As the test results, the following conclusions were obtained. 1) Stress-strain response for step tests, block loading and block random loading were different from that for constant stress amplitude, and cyclic softening for variable stress amplitude was greater than that for constant stress amplitude. 2) Especially cyclic softening under variable stress amplitude was remarkable for low stress level. 3) Fatigue life estimation by Miner's rule makes over estimation compared with the experimental values, and fatigue life of block random loading is slightly shorter than that of block loading under the same stress frequency distribution. 4) When the plastic strain amplitude was used as a determining factor of fatigue damage, the estimated fatigue lives fairly approached the experimental values.