For the purpose of establishing the crack propagation law we make a consideration about the small change of compliance for a cracked body under cyclic loadings. From the consideration, it becomes clear that the load just when re-tensile plastic zone generates at a crack tip under loading process is little higher than crack opening load. In this loading range, the cracked body shows fully elastic and then it is expected that no damage occures in the vicinity of the crack tip. From the fact, we propose effective stress intensity factor range corresponding to the terms of appearing re-tensile plastic zone (Δ K_RP ) in place of Δ K _eff proposed by Elber as for a fatigue crack propagation parameter. Moreover for the purpose of measuring the re-tensile plastic zone's generating load (RPG load), a subtraction circuit which can measure the small change of compliance during fatigue test is developed and automatic controlled system which can controll the adequate value of resistances in the circuit and the output voltage range from strain amplifiers for minimizing relative noize level is also developed. CT test results by using the newly developed system show that RPG load is higher than crack opening load but is lower than crack closing load.