Knowledge of resin chemical shrinkage is crucial for the determination of residual strains, stresses and warpage of composite parts during curing. Shrinkage measurement is more accurate on several millimetre thick samples. However, in that case thermal properties of resin and the strong coupling between thermoset chemical reactions (generally rapid and strongly exothermal) and thermal fields lead to non-negligible thermal and curing gradients in the piece. It is then necessary to take these variations into account to have an accurate description of the shrinkage. In the present study, a home built device "PVT-α" mould is used to measure the volume variation of vinylester resin and associated composites during the curing and then shrinkage is identified by considering these gradients. The results demonstrate that a linear evolution of the shrinkage with conversion degree is a good model to describe the chemical effect on the global volume behavior of the piece. The contribution of chemical and thermal effects on volume curve is quantified.