Abstract: A leaky-wave antenna with low sidelobe property is proposed based on the leaky rectangular waveguide.The leaky-wave antenna is first equivalent to the line-source model and then the segments of the line-source that mainly contribute to the far-field radiation are figured out according to the theory of the “effective radiation segments”. The low sidelobe property of the leaky-wave antenna is realized directly by narrowing the corresponding slot segments of a sidelobe and indirectly by widening the corresponding slot segments of the main lobe respectively. Both of the two methods can reduce the sidelobe level at different extents, at most by about 11. 5 dB at 10 GHz, which is verified by CST simulations.