Abstract: An optical sub-assembly module includes a receptacle combined with a light emitting device. The receptacle includes an optical axis, a coupling structure is aligned with the optical axis. The light emitting device includes a outer seal and a light emitting chip disposed in the outer seal. An open aperture is formed in the surface of the outer seal and opposite to the light emitting chip, and a transparent surface of the light emitting chip is a flat structure without lens. Therefore, the light beam emitted from the light emitting chip does not pass through any micro lens, and the light beam between the light emitting chip and the coupling structure is not refracted.