The frequency dependence of helicon wave plasmas has been studied at 1-25 MHz of rf power, and at magnetic fields of 200-1150 G using various gases. Maxima in the electron density occur for particular frequencies at each magnetic field. These optimum frequencies are propertional to magnetic field strength and inversely proportional to the discharge gas mass. The existence of optimum frequencies in each magnetic field and gas mass indicates the possibility of lower hybridresonance heating effect. The experimental optimum frequencies are slightly lower than the theoretical lower hybrid resonance frequencies. The difference between the two frequenciescies can be explained by the Doppler shift effect.