The fiber optic sensor system for chemical vapor detection was desiged and constructed. The system consisted of three parts; the optic unit, the fiber-optic sensing head and the flow controlling unit. The optic unit included a He-Ne laser source which lazes a red laser into an aligned optical fiber, a photo detector, and a signal processing with computer interface controlled by the Labview® program version 7.1. The sensing head was made of a polyaniline thin film coated onto the de-cladded section of an optical fiber covered by a gas mixing cell. The concentration of measured gas was controlled by varying nitrogen gas flow rate. The nitrogen flow controller was set-up to obtain vapor concentration in the range of 0.04 to 0.40 % v/v. Vapors of hydrochloric acid (HCl) and n-butyl amine (a weak base) were used to test the performance of the sensor system. It was found that output intensity increases with an increasing HCl concentration and decreases with increasing n-butyl amine concentration. The response toward the amine vapor was faster than that of the HCl vapor (23 seconds for n-butyl amine and 72 seconds for HCl). Experiments performed at various concentrations of amine vapor (between 0.04 to 0.21 %v/v) found that a higher concentration yields faster response time.