This Paper builds an international trade model of endogenous innovation more consistent with empirical evidences on R&D productivity and theoretically revisits how international trade affects economic growth. Specifically, to reconcile the shortcomings of the conventional model with the empirical evidences of both decreasing returns to R&D and the limited international knowledge spillovers of international trade on R&D productivity, as well as the costly adoption of technology to the production process, this paper makes the R&D productivity subject to the congestion externality from previous R&D and also introduces the technology specific training for production workers as the firm’s investment for the adoption of technology. Main findings are as follow. Trade affects growth positively, and the mechanism for this positive effect is the resource allocation rather than the costless knowledge spillover. Thus, the greater the dissimilarity among trading countries, the greater the growth effect, but trade between countries with the identical economic structure has no growth effect.