A new design methodology for dual band leaky-wave antennas is presented in the paper. The proposed methodology is based on the use of a single partial reflective surface (PRS) made with a Frequency Selective Surface (FSS) that fulfills the resonant condition at two different frequencies. This allows the design of a dual band leaky wave antenna including the control on the directivity at each one of the frequencies. Two examples of the application of this method (one with identical directivities at the two frequencies and the other one with different directivities) are presented.