Palm oil is one of common consumed raw food and found adulterated with another materials such as lard. Palm oil adulteration can cause some problems in norms and health. Some methods have been developed in detect lard mixing in foods such as spectroscopy based methods, including infrared spectroscopy and some methods which is complex, expensive, and need of experts. In other side, plastic optical fiber with u-shaped configuration has been developed as biosensors. Through utilization of evanescent field and effect of the change in refractive index to the transmitted power, it has been design a sensor based on plastic optical fiber to detect lard adulteration in palm oil. Experiments show that the most optimal sensor design based on plastic optical fiber with u-shaped configuration is sensor with Infrared LED as light source, 50 mm curvature radius of bend, dan 2 cm length of peeled cladding. From the experiments, sensitivity of the sensor is 0.15630352 μW / % lard concentration.