Abstract: On the basis of separation crack-arrest principle, a special chary impact specimen was designed for the quantitative evaluation on embrittlement behavior of fusion zone in dissimilar steel weleded joints by using Charpy Instrument Tester. The fracture mechanism was studied with SEM. The effects of different butter layers and PWHT parameters on embrittelment and fracture behavior of fusion zone were also discussed. The results indcated that embrittlement of fusion zone as welded was caused by the martensite, and the fracture morphology was cleavage in martensite layers. After PWHT the embrittelment was mainly caused by the carbon enriched/depleted layers, and the fracture morphology was carbide and/or an intergranular fracture. There was no significant influence of different butter la yers on the embrittlement and fracture mechanism, but great influence of different butter layers on the embrittlement grade existed.After PWHT the embrittlement index (Eb)of fusion zone for HD309L butter layer was higher than that for HD457L butter layer by one and half times, and by four times as welded.