Ammonia (NH3) is an important industrial raw material and a harmful gas for humans. Therefore, fabricating highly performance NH3 sensor is very significant work, which is reliable, inexpensive and operating at room temperature. The NH3 sensor materials include metal oxides, carbon nanotube and conducting polymers. Among the conducting polymers, polyaniline (PANI) is one of the most appealing sensing material due to its ease of synthesis, environmental stability, reversible redox reaction and low energy consumption (operating at room temperature). In order to enhance the mechanical strength and characteristics of sensors, inorganic as nanofiller (such as TiO2, SnO2, MoO3, In2O3, carbon, etc) is combined into PANI material to form nanocomposite. Nanodiamond (ND) has firstly been described in the 1960s by Russian researchers. Compared to bulk material, ND can offer larger specific surface area, higher chemical stability and higher mechanical modulus. The ND is homogeneously combined into PANI to form ND/PANI nanocomposite for NH3 gas detection, which may increase the specific surface area and improve the sensitivity of ND/PANI sensor.