This paper were performed to evaluate the elastic-plastic fracture toughness $J_IC$ by the unloading compliance method for 1/2 CT specimen of A5083-H112 aluminum alloy. A modified offset method for negative crack growth effect is presented and compared with Voss offset method. We found that the modified offset method presented can be used efficiently for a determination of the stable crack onset. The $J_IC$ by Voss offset method can be overestimated, but the suggested offset method showed a high reliability within 5% of tolerence from the results by the multi-specimen method.