A new miniature interferometer has recently been developed for monitoring the changes of film thickness and refractive index, wherein a semiconductor laser and a photodiode detector are integrated to form a laser/detector hybrid. The laser light is collimated by a lens. When the laser light hits a film, it is reflected twice by the two interfaces between the film and the surrounding mediums. The two reflected light beams are interfered at the detector by the same lens. Due to the interference, the detected intensity is a function of the film thickness. The swelling of thin gelatin gel film has been used to demonstrate the use of this novel sensor. The sensor is ready to be used in many applications, such as inside a vacuum chamber, on a robot arm, and in a small reaction container, wherever a conventional interferometric setup cannot be easily implemented because of its size.