Raman microprobe spectroscopy was used to characterize in situ microstress fields which develop during fracture of a toughened silicon nitride (Si3N4) polycrystal. Maps of microscopic stress were collected in the neighborhood of a propagating crack both at zero and at critical loading conditions. Micromechanics results by Raman spectroscopy were analyzed and compared with conventional fracture mechanics assessments, such as the evaluation of rising R-curve behavior and crack opening displacement. Outcomes of these assessments illustrate that, despite the approximations involved in the piezo-spectroscopic equations used for calculating the microstress field from a local Raman shift, Raman microprobe spectroscopy is a viable method for semi-quantitative investigations of microfracture mechanisms in advanced ceramic materials. Copyright (C) 2002 John Wiley Sons, Ltd.