AbstractTechnology spillover and research and development (R&D) budget are relevant on government subsidies that aim at improving social welfare through enhancing R&D incentives of firms. However, there has not been related literature considering these two factors. To fill this gap, this paper examines the effect of technology spillover and R&D budget on R&D competition of duopolistic firms and government subsidies by constructing a game‐theoretic model. We find that while each firm's profit sometimes increases with R&D budget for low coefficient of technology spillover, this profit may decrease with R&D budget for high coefficient of technology spillover due to the intensified R&D competition. We show that when both R&D budget and the coefficient of technology spillover remain high, R&D subsidy leads to higher social welfare than output subsidy and otherwise R&D subsidy results in lower social welfare.