The effects of the crystallization behaviour of CaMgSi2O6 (diopside) glass-ceramics on their microwave dielectric properties were investigated as functions of the Cr2O3 content and heat-treatment method used (one or two steps). The crystallization behaviours of the specimens were affected by the Cr2O3 content as well as by the heat-treatment method employed, and were evaluated using X-ray diffraction and the combined Rietveld and reference intensity ratio (RIR) method. The dielectric constants (K) of the specimens did not change significantly with an increase in the Cr2O3 content. The quality factor (Qf) of the specimens increased for Cr2O3 contents of up to 0.5wt% Cr2O3, but then decreased for higher contents. These results could be attributed to the degree of crystallization. For the same Cr2O3 content, the specimens that underwent a two-step heat treatment showed lower K values and higher Qf values than those heat-treated in one-step. These results could be attributed to the smaller crystallite size and higher degree of crystallization in the specimens obtained from the two-step heat treatment compared with those of the specimens heat-treated in one-step method.