Disclosed are a heterogeneously integrated optical modulator and a manufacturing method thereof. The modulator includes a substrate having a trench, an input waveguide disposed at one side of the trench, an output waveguide disposed at the other side of the trench, a first Mach-Zehnder interferometer including first branch waveguides disposed between the input waveguide and the output waveguide and a heater disposed on one of the first branch waveguides, and second Mach-Zehnder interferometers connected to each of the first branch waveguides.