The presence of mercury ions is a serious threat to human health and environment. Water consumption containing mercury is very dangerous for human healt. The purpose of this work is to design a heavy metal sensor  ion using plastic optical fiber to detect mercury ions in water. The sensor was fabricated by coating the optical fiber by chitosan through dip-coating technique. There are three diameter of optical fiber sensor that was fabricated, which are 1087.64 μm (sensor A), 1691.64 μm (sensor B) and  1736.33 μm (sensor C). Sensor characterization was done by connected the tip of plastic optical fiber to an LED and the other tip was connected to a photodioda. The results show that sensitivity of sample A is 0.32 mA/ppm, sample B is 0.56 mA/ppm and sample C is 0.64 mA/ppm. In terms of respone time, it is shown that  thr respon time for sensor A, B and C are 22 s, 42.5 s and 86 s respectively.