This paper considers a duopoly with product differentiation and examines the interaction between merger and innovation incentives. The analysis reveals that a merger tends to discourage innovation, unless the investment cost is sufficiently low. This result holds irrespective of whether side payments between firms are allowed. When side payments between insiders within the merged firm are permitted, a standard result in the literature is reconfirmed, which suggests that a bilateral mergerto-monopoly is always profitable. When such side payments are not permitted, however, it is shown that a merger is not profitable when the efficiency of the new technology is relatively high and the investment cost is below a particular level.