A chemical sensor comprised of a thin layer of polyvinylidinone (PVP) -carbon black (CB) composite on interdigited electrode (IDE) was fabricated and tested. The thin composite film was fabricated by spin casting an aqueous solution of PVP (30 mg/ml) which contained CB in suspension form. The percolation threshold for this system was investigated by varying the amount of CB in the composite and it was found to be about 30% by weight of CB. The effect of CB content on sensor sensitivity to chloroform was also studied. The results indicated that the sensor with 35% CB composite exhibited highest sensitivity while the sensors with 30 and 40% CB gave five times lower sensitivity. Therefore, the sensor with 35% CB was used for the rest of studies, i.e. the effect of nitrogen flow rate which utilized as a carrier gas, chemical vapor exposure time and types of chemical vapor. We discovered that the sensor gave not only better response but also faster recovery time when a higher carrier gas flow rate was used. The sensor sensitivity presented a linear relationship with exposure time in a limited region before the sensitivity became saturated. Currently, two chemicals vapors, i.e., chloroform and ethanol were studied and the result shown that the sensor exhibited much higher sensitivity to chloroform than that to ethanol at the same condition.