The formation and decomposition of MC carbides in alloys with different contents of Re and Ta were investigated by DSC, SEM, EDS and XRD. The results show that script MC carbides in three alloys was formed by eutectic solidification. The addition of Ta not only decreased the W concentration but also minimized the difference of W composition in MC carbides formed at difference freezing period in three alloys. Meanwhile, the addition of Ta also increased the formation temperature and the thermal stability of MC phase significantly. Re was not found in MC carbides. The addition of Re slightly decreased the formation temperature of MC carbides and promoted the precipitation of μ phase from the oversaturated γ matrix.