We consider a semi endogenous R&D growth model with international trade, firm heterogeneity, and local knowledge spillover in a closed economy and international knowledge spillover in a symmetric two country economy. We show that by opening trade R&D difficulty (the number of varieties produced) and welfare are ambiguously affected. When the international spillover is large (small), the former is increased (decreased). When the size of the international knowledge spillover is large (small) or the size of the international knowledge spillover is small and the size of intertemporal knowledge spillover is small (large), the latter increases (decreses). Without intertemporal and international knowledge spillovers, welfare increases.