This research aims at studying the structural refinement of 15%Cr-2%Mo hypo- and hypereutectic white irons through dynamic solidification and Nb-alloying (1-2%Nb). Solidification behavior was studied using DSC. Microstructure investigation and analysis were performed using optical, image analyzer as well as SEM, EDX and XRD analysis. Refinement mechanisms proposed involved both enhancing nucleation and altering the growth process of the primary and eutectic phases either by fragmentation of the primary growing phase in the case of dynamic solidification or by consuming the available carbon from the liquid and hence the amount and size of M7C3 will decrease in the case of addition of Nb-alloying. Nb has higher affinity to carbon during solidification, higher formation temperature than the main microstructure constituents and forms fine carbide so Nb achieves refinement of the microstructure.