Thermal shock resistance of silicon nitrides is investigated using an indentation-quench method. Four commercially available silicon nitrides with different microstructures are investigated. The extension of Vickers radial cracks is measured as a function of quenching temperature for each material, up to the critical temperature for failure. An indentation fracture mechanics analysis is used to account for the crack responses, with due allowance for R-curve behavior. The analysis confirms the important role of microstructure in thermal shock resistance.