A L-band three-way power combiner with planar structure was proposed. The combiner was implemented by the microstrip circuit composed of two sections of three-way quarter-wavelength impedance transformers and planar isolation resistances. The final design was obtained by simulation and optimization. The experimental results were in good agreement with the simulation analysis. In the bandwidth of 1.1 GHz-1.7 GHz，the Voltage Standing Wave Ratio(VSWRs) of four ports showed less than 1.13 and the isolation among three ports showed more than 25 dB. A minimum combining efficiency of 94.5% was measured with a maximum amplitude imbalance of 0.05 dB over the 1.1 GHz to 1.7 GHz band. The performance of the power combiner with compact structure meets the design requirements.