This paper studies the effects of transversallyloaded slot and longitudinally-loaded slot on the microstrip second higher order mode leaky-wave antenna from a circuit perspective. The transversally-loaded slot array introduces a radiation beam with θ-polarization through the enhancement of the cross polarization, while the longitudinally-loaded slot array can effectively reduce the dual I-polarization radiation beams due to the field cancellation. As a result, the single radiation beam can be obtained when the I-shaped slot array is mounted along the microstrip leaky-wave radiation strip under the second higher order mode excitation. As a model analysis, the equivalent πcircuit is established and extracted to understand the loading effect of periodic properties. The results show that the transversal slots mainly work as a series impedance, while the longitudinal slots contribute to the shunt admittance. The obtained circuit model successfully explains the physical insides of the propagation constant and the radiation characteristics.