Considered a differentiated duopolistic model where the two firms both conduct R&D activities. The paper studies the optimal technical spillovers for R&D under four cases which three different kinds of cooperation modals about maxim of firms’ profits and one about maxim of social welfare. And the effects of change of optimal technical spillovers on output, profits, consumer surplus are analyzed under every case. And results show that the optimal technical spillover is one when cooperation exists in R&D stage and it could not only be one when cooperation doesn’t exist in R&D stage.