A semi‐analytic method is presented for dual‐band power amplifier design. A commensurate transmission line is applied to reduce the number of independent variables in a given topology so that the optimisation algorithm would be more robust to handle it. At the same time, it could be described as a rational function. To pursue a good performance at the two bands, continuous mode is also considered for controlling the harmonics. The impedance of the lower band is controlled up to the third harmonic, while that of the upper band is up to the second harmonic. The efficiency of the dual band is, respectively, 69.4 and 70.7%, and the corresponding output power is 41.7 and 41.5 dBm at 2.4 and 3.45 GHz, respectively.