A frequency modulator basically consists of an amplifier and a resonator employing a varactor diode, and the linearity of frequency modulators has been analyzed and discussed looking at the resonance frequency calculated from the capacitance-voltage characteristics of a varactor diode. However, as the oscillation frequency of the frequency modulator increases, it becomes essential for designer to evaluate the influences on the modulation linearity by the phase characteristics of the amplifier. This paper discusses the influences on the modulation linearity by the phase characteristics (the phase slope (or group delay) and the cross frequency fc) and by the capacitance-voltage characteristics of varactor diodes, and the design of a UHF frequency modulator utilizing a hybrid integrated amplifier, is described. It is pointed out that, although the capacitance variation ratio n(log C(V)/log V) is less than 2, the capacitance variation ratio increases with the varactor diode bias voltage so that good modulation linearity can be obtained. Furthermore adopting a push-pull mode of operation, two UHF frequency modulators are connected to obtain a wideband frequency modulator (the bandwidth is f025 MHz). Finally a feedback-type frequency modulator operating at 6 GHz is discussed, as comparison to the result of the UHF frequency modulators.