Abstract: From the concept of fracture work, a new method has been developed to evaluate the toughness of welding fusion region for dissimilar metallic materials by means of "Implant" tost. For 21/4Cr1Mo steel/stainless steel, the experimental results indicated that the relationship between the fracture work W_f and the Charpy impact work A_K follows the equation A_K=1.17W_f+0.94. The toughness of welding fusion region, is increased with the decrease of the width of bunchy M-B Structure region.