This study examines the R&D risk choice in a duopoly market with technology spill overs. The firms conduct R&D programmes with different degrees of risk but an identical expected outcome and they compete or cooperate in R&D. Findings indicate that, in equilibrium, the R&D risk level decreases in the spill over rate under noncooperative R&D, while it may increase under cooperative R&D. Firms are more likely to engage in higher R&D risks under cooperative R&D than they are under non‐cooperative R&D. Moreover, the equilibrium R&D risk level both under competition and cooperation R&D is always too low from the perspective of social welfare, and the extent of this inefficiency increases with the spill over rate if the size of the spill over is large, but the opposite may occur if the size of the spill over is small.