A periodically loaded leaky wave antenna based on composite right/left handed substrate integrated waveguide (CRLH SIW) is proposed in this paper. The series capacitance contributed by etching periodically transverse slots in the upper side of the waveguide and shunt inductance contributed by metallic inductive multiple posts shorted to the ground in the rectangular SIW results a transmission line with left-handed property. With this configuration, a frequency scanned leaky wave antenna is designed by cascading 13 unit cells. Frequency responses and radiation patterns of the compact structure are measured and the experimentally observed results confirm the expected scanning leaky wave behavior of the proposed periodic CRLH SIW.