2.5Gb/s optical transmitter IC for vertical-cavity surface-emitting lasers (VCSELs) was fabricated using the TSMC 2-poly 4-metal process of 0.35μm Si-CMOS. This IC has four channels with 250μm pitch for VCSEL array. The transmitter IC consists of four parts, i.e., buffer, differential amplifier, threshold current controller and modulation current controller. Modifying the threshold current and modulation current, we attained a wide range of the operation current from 1mA to 20mA. The measurement of the IC chip packaged with a 850nm VCSEL array, we obtained a maximum optical output power of 2dBm in 2.5Gb/s operation and an extinction ratio of 5~25dB.