In this paper, an on-chip all digital temperature sensor using dual ring oscillators which have different gate channel length is proposed for maintaining the performance benefit of CMOS digital circuit. This novel temperature sensor measures the temperature variations between the temperature-sensitive ring-oscillator and the temperature-insensitive ring-oscillator according to temperature. The sensitivity of delay variations depends on the CMOS gate channel length. Output clock of the ring-oscillator which consists of the long gate channel length inverters is sensitive to temperature variation. In contrary, output clock of the ring-oscillator which consists of the short gate channel length inverters is insensitive to temperature variation. The difference between two clocks can be amplified using counters. Finally, it is possible to sense the temperature variation by using coarse-fine TDC(Time-to-Digital- Converter). The circuit was designed by 0.11μm 1P6M process. Effective resolution was 1.0°C from 0°C to 100°C and its chip area was only 0.043mm2 with 1μW power consumption at 1.2V operation voltage.