Optical losses of silica waveguides are mainly caused by the core sidewall angle and rough. ness of the silica waveguides. In this study, to accurately measure the sidewall angle of an upper-clad. ded waveguide core, we investigate a method using an optical image microscope. Further, we thor. oughly analyze the relationship between the magnification of the microscope objective and the imaging features that cause measurement errors. Then, we compensate the experimental measurement results of the waveguide sidewall angle with the intrinsic systematic errors based on a theoretical analysis, which agree with the values obtained via confocal laser microscopy measurements. Furthermore, by comparing the sidewall angle measurement results of 10 sets of cladded waveguides with the confocal laser microscopy measurements of the waveguides before being cladded, we demonstrate that the measurement error is controlled within ±1°. This provides a feasible method to quickly and accurately measure the sidewall angle of the core layer of cladded silica waveguides, i. e., after deposition of the cladding.