A method for crystallizing a silicon substrate includes manufacturing a crystallized silicon test substrate that is crystallized by scanning excimer laser annealing beams with different energy densities on respective areas of an amorphous silicon test substrate, irradiating a surface of the crystallized silicon test substrate using a light source, and measuring reflectivity corresponding to the respective areas of the crystallized silicon test substrate in a visible light wavelength range, extracting average reflectivities of the respective areas of the crystallized silicon test substrate in wavelength ranges corresponding to respective colors, calculating an optimum energy density (OPED) index per energy density by using a value acquired by subtracting average reflectivity of red-based colors from average reflectivity of blue-based colors, selecting an optimal energy density, and crystallizing an amorphous silicon substrate using the optimal energy density.