This paper attempts to find a suitable technique for determining prior austenite grain size in mild steel such that it could be used over as wide a range of compositions and temperatures as possible. An effort is also made to trace the reasons behind the failure of any particular technique when applied beyond a specific temperature and composition range. Plain carbon steels, steels with Nb, V, Al, N additions, and steels containing a combination of two or more ofthese elements were studied between 900–1250°C. The investigated methods are applicable to determining austenite grain size developed in the absence of deformation.