This paper studies the impact of a fiscal stimulus by setting up a simple monopolistic competition model with firm heterogeneity in productivity. Several main results are derived from the general equilibrium analysis. First, a rise in firm heterogeneity per se leads to decreases in aggregate output and aggregate consumption, but raises the aggregate price level when the variety-enhancing effect is sufficiently strong. Second, a fiscal expansion will bring about a positive effect on aggregate consumption, provided that the variety-enhancing effect is relatively strong or the extent of firm heterogeneity is relatively small. Finally, a fiscal expansion may raise social welfare, depending on the size of the variety-enhancing effect and the extent of firm heterogeneity.