A 5 GHz-band direct digital radio frequency (RF) modulator is proposed and fabricated in 90-nm complementary metal oxide semiconductor (CMOS) process. Since this modulator directly converts digital base-band (BB) parallel input signal into RF signal, small die size and low d.c. operation are achieved. It consists of an inverter section to obtain digital differential BB signal, a differential current-mode digital-to-analog converter (DAC) section and a local oscillator (LO) switch section. The differential DAC configuration enables low glitch performance at the BB current output and low spurious emission at the RF output. The fabricated direct digital RF modulator performs RF output power of -38.6 dBm with LO leakage of -88.1 dBm at 5GHz LO and 1-MHz BB signals. The core size of fabricated integrated circuit (IC) is 200μm × 170μm and d.c. power consumption is 2.5 mW (2.1 mA / 1.2 V).