A projector and a wavelength conversion device thereof are provided. The wavelength conversion device includes a wavelength conversion element and a diffusion element. The wavelength conversion element includes a first substrate and at least one wavelength conversion layer. The at least one wavelength conversion layer is disposed on the first substrate and surrounding an axle center of the first substrate. The at least one wavelength conversion layer is configured to perform a conversion on a light beam. The diffusion element is connected to the first substrate of the wavelength conversion element and surrounding the axle center of the first substrate. The diffusion element is configured to allow another light beam to pass through.