A voltage-controlled oscillator (VCO) is one of the most important basic building blocks for analog and digital circuits. There are different implementations of VCOs. One of them is the ring oscillator based VCO. A ring oscillator based Temperature Sensors designed is sensitive for small variations of temperature. The Temperature is linearly dependent on the oscillation frequency of ring oscillator,as a way of converting temperature to a digital read out. The proposed temperature sensors occupies smaller silicon area with higher resolution than the conventional temperature sensors based on band gap reference. It was designed to achieve better than 1^o C of temperature resolution, which is suitable for almost any application of an integrated CMOS temperature sensor. Special attention has been given to the integration of design of two different Ring Oscillators in order to achieve lower voltage operation when the design is implemented in an advanced CMOS process.