Provided is a sub-harmonic frequency mixer having a structure in which two terminals of transistors receiving a Radio Frequency (RF) signal and a Local Oscillation (LO) signal are coupled. The frequency mixer is composed of a single-level structure of transistors and thus can be driven at a lower supply voltage compared to a common frequency mixer. A direct-conversion receiver employing such a frequency mixer needs an RF signal and an LO signal sources of single-phase. Therefore, the direct-conversion receiver has an architecture that simplifies a whole RF transceiver circuit and thus can be advantageously applied in implementing SoC (System-on-Chip) for a low power, high integration, low price and subminiature wireless transceiver circuit.