Abstract : This article aims to analyze the effect of debt on the profitability. This impact can be explained by three essential theories: signaling theory, the agency costs theory and tax theory. The sample consists of a panel of 1,078 French firms, unlisted, anonymous type and Limited Liability Company; belonging to the construction sector over the period 1999-2006. We show, by using the generalized method of moments (GMM), that the debt affects negatively the profitability, not only linearly, but also, in a non-linear (concave) way. However, by detailing the analysis according to different class of size, we find that the linear negative effect and the non-linear effect are significant only in small and medium-sized enterprises (SME).