Glass manufacturing involves abundant amount of natural resources including thermal energy and it is important to explore various ways to ultilize the used glass instead of direct disposal to landfill. One way to recycle the glass products is to crush them into cullets and mix the cullets into cement as filler/aggregate. In this study, the effects of recycled glass cullets (RGCs) with different sizes on the fire resistance performance of cementitious composite were studied. Cementitious composite samples were prepared with RGCs at 4 different sizes, i.e. below 0.6 mm, 0.6–1.18 mm, 1.18–2.36 mm and 2.36–4.75 mm. In order to evaluate the fire resistance of the composites with RGC, the composites samples were heated at different temperatures, and their residual strength, integrity and thermal conductivity were measured accordingly. Moreover, a specially designed furnace in accordance with modified BS 476-22 was employed to observe the thermal insulation and integrity of mortar panels under testing. The microstructure of RGCs mortar was observed under microscope and it was found that adding RGCs inside the composite improved the fire resistance of mortar. The beneficial effect of RGCs on fire resistance was found to reduce with increasing RGCs size, where RGCs finer than 0.6 mm showed the most significant beneficial effect. This is presumably attributed to the softening and state transformation of fine glass cullet at high temperature.