We report a simple approach to measure relative humidity with high dynamic range and fast response time using Cobalt chloride (CoCl 2 ) doped polymer thin film. Anhydrous Cobalt chloride exhibits high absorption at low humidity level in the wavelength range 550-700 nm while possess very low absorption at high humidity level present in the environment . Thin CoCl 2 doped polymer films of different thickness were fabricated on thin cover slides using dip coating technique. Expanded light from a diode laser source ( 680 nm ) was allowed to transmit through the sensing film of the slide. The variation in intensity of the transmitted light with the moisture level present in the environment was monitored using silicon photodiode and finally the detector signal was taken through a computer. As large area of the sensing region is exposed to the environment, high sensitivity of the sensor is achieved . Further, sensor response was investigated in terms of chemically synthesized film thickness on the slides. A high dynamic range of the sensor is obtained with fully reversible and repeatable behavior. The advantage of the system is its simplicity and is a very low cost technique to monitor relative humidity present in environment .