Recently attempts have been made by several researchers in order to develop biodegradable composites. In the present work hybrid composites of unsaturated polyester based sisal/glass fibre hybrid composites were prepared. The sisal is a natural fibre which is also biodegradable and glass fibre is a synthetic fibre. These two natural and synthetic fibres are combined in the same matrix (unsaturated polyester) to make sisal/glass fibre hybrid composites and the heat capacity of these hybrid composites was studied. A significant improvement in heat capacity of sisal/glass fibre hybrid composites has been found. The chalk powder (additive) is also added to the resin (unsaturated polyester) in proportions of 1%,2%,3% by weight of resin respectively and sisal/glass fibre hybrid composites were prepared by using this resin to study the effect of chalk powder on heat capacity of these hybrid composites. It is also observed that as the chalk powder quantity increases the heat capacity also increases.