A new technique using mixed frequency transformations has been proposed for wideband bandpass filter design to suppress the harmonic passband. Different modified Richard's transformations (MRTs) are used herein to maintain an almost exact in-band transformation but different out-of-band characteristics. Thus, the original in-band filtering response can be transformed through a uniform pattern, while the harmonic resonance of every single resonator is relocated at some desired places. The synthesis procedure of the filter includes deriving the equivalent LC circuit, calculating MRTs, transforming the LC circuit into its transmission line counterpart by mixed MRTs. A design example has been demonstrated to strongly support the proposed technique in the experiment.