In the large field of nanotechnology, polymer matrix based nanocomposites have become a prominent area of current research and development. This book deal with the fabrication of nanocomposite (PANI_CNTs nanofiber) by electrochemical method for NH3, HCl, and H2 sensors with described the mechanism of its interactions. The effect of weight proportion of CNTs to polyaniline to making nanofiber composite on sensitivity and response time and recovery time are described.