This paper presents a simple architecture for 8-bit digital controlled oscillator (DCO) on 3-stages ring topology in TSMC 0.18um CMOS technology. A new schematic of tristate inverter is also proposed. The proposed tristate inverter has higher switching speed and low power consumption as compared to conventional one. The control digit changes the driving current that provides large tuning range from 316 MHz to 1165 MHz with very good linearity. The output voltage swing is always greater than 1.6V while the supply voltage is 1.8V. The proposed DCO has simulated phase noise of -114dBc/Hz @1MHz and 23.2mW power consumption at 1.0 GHz central frequency.