We have realized $Si/SiO_2$ microcavity consisting of λ/2 SRSO active region deposited by ECR-PECVD system and two $Si/SiO_2$ distributed Bragg reflectors deposited by sputter system. Photoluminescence at room temperature reveals an enhancement of the luminescence intensity emitted along the optical axis of the cavity, and a decrease of the FWHM of resonant peak. The intensity of the cavity structures is typically one order of magnitudes higher as compared to structures without cavity. Furthermore, the emission wavelength and the intensity decrease for off-normal detect angles. The change in emission wavelength can be quantitatively described by assuming the on-axis component of the optical wave is resonant with the cavity. These PL enhancement, spectral sharpness, and directionality of the microcavity demonstrate the very high potential of $Si/SiO_2$ microcavities for optoelectronic applications.