An optical device includes a phosphor wheel and two light sources. The phosphor wheel has two phosphor regions. The phosphor regions are located at different radial positions of the phosphor wheel and are not overlapped. Each of the phosphor regions has a plurality of color sections. The light sources emit two light beams so as to respectively provide two light spots on the phosphor wheel. During the rotation of the phosphor wheel, the light spots are located at the color sections having the same fluorescent characteristic respectively in the phosphor regions.