A device and method for synthesizing a transmission frequency (a Tx frequency) in a wireless terminal are provided, which are designed for stably operating a Phase Locked Loop (PLL) in order to synthesize a Tx frequency for a certain channel. The device includes a Radio Frequency (RF) transceiver for operating a Phase Locked Loop (PLL) in order to synthesize the Tx frequency when the wireless terminal requires the Tx frequency, and for transmitting the Tx frequency when the PLL is stabilized, a first signal generator for providing the RF transceiver with a first signal until the PLL is stabilized, a second signal generator for providing the RF transceiver with a second signal in a duration, for which the first signal is not provided, until the PLL is stabilized, and a controller for controlling the first signal generator to provide the RF transceiver with the first signal until the PLL is stabilized, and for controlling the second signal generator to provide the RF transceiver with the second signal in the duration for which the first signal is not provided.