Dynamic deformation characteristics such as dynamic yield stress and dynamic strain hardening exponent along with dynamic elastic-plastic fracture toughness for the different microstructural regions of heat affected zone (HAZ) of a nuclear grade 9Cr-1Mo steel have been evaluated by instrumented Charpy tests. Isothermal heat treatment at different temperatures was used to simulate the different microstructural regions in the HAZ of this steel, namely the over tempered base metal, intercritical, fine prior austenitic grained martensitic, and coarse prior austenitic grained martensitic regions. Effects of interparticle spacing on the dynamic deformation and fracture properties have been studied. It has been observed that the dynamic yield stress and the dynamic fracture toughness follow power law relationships with the interparticle spacing whereas the strain hardening exponent follows a linear fit.