An optical pressure sensor utilizing a silica-based GRIN rod lens is proposed and demonstrated. The major advantage of the sensor is inexpensive system configuration whose main part consists of low-cost components such as the rod lens, an LED light-source and photodiodes. The sensor also has two other superior characteristics. One is stability because the silica-based rod lens can be attached to the optical fiber directly by fusion splicing without adhesive or other materials. The other is flexibility because the sensitivity of the sensor can be varied by changing the rod lens length. In proposed method, optical power variations correspond to a displacement of a bellows is measured to detect the pressure. We have made two optical pressure sensors with different length rod lenses. Each lens is designed to measure up to 200 kPa and 10 kPa pressure. The displacement amounts of the bellows due to these pressure levels are 600 μm and 30 μm, respectively. The +/-0.2% accuracy of full scale is achieved for both sensors and +/-60nm displacement can be measured by this simple method.