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The additions of 0·022–0·025 mass- phosphorus and 0·010–0·012 mass- boron could enhance the creep properties and increase the service temperature of IN718 superalloy markedly. However, the as cast microstructure and solidification behaviour of modified IN718 with P and B additions are not completely recognised. In the present paper, differential scanning calorimeter (DSC) technique accompanied by scanning electron microscope and electron probe microanalysis characterisation is applied to understand the effects of P and B additions. It is found that P and B additions may promote the formation of big blocky Laves phase and result in the existence of B bearing phase enriched in Nb and Mo. In the meantime, another endothermic peak appearing before the Laves phase peak in the DSC heating curve is related to the melting of the B bearing phase. After the thermodynamics calculation, this new endothermic peak detected may be involved with the melting of M3B2. Furthermore, the endothermic peak appears before the Laves phase peak in the DSC heating curve, suggesting that the incipient melting temperature is greatly reduced as a result of P and B additions. In addition, both melting and solidification temperature ranges are significantly widened, indicating that the solidification process becomes more complex for the P and B doped IN718 superalloy.