A transmission trimming method for Si waveguides using phase change material (PCM) is proposed. We used a 3D-finite difference time domain (3D-FDTD) method to calculate the phase and the loss of light induced by the trimming structures. With five small square patches of PCM on the waveguide, transmission trimming up to 32 levels can be achieved. It was found that the crosstalk of a MZI type optical switch can be reduced when the splitting ratio of the first coupler is corrected using this method.