We demonstrate that the coherent control of unimolecular reactions by using a waveform-controlled laser fields can lead to a strong modulation on the yield of the reaction. By using a synthesized ω (1800-nm) and 3ω (600-nm) two-color laser field, the probability of photodissociation of H2+ can be strongly modulated by varying the relative phase between the two colors. The dissociation probability maximizes at different relative phases for protons with different kinetic energy, and such energy dependence can also be qualitatively reproduced by our simulation. We attribute the observed dissociation probability modulation to the interference between two different dissociation pathways which start from the same electronic states and end with the same kinetic energy.