This article describes the design and testing of a laboratory prototype single diode laser system for remote detection of methane gas using harmonic detection. Harmonic signals are contained within radiation backscatter from topographic targets. A function of detected gas concentration depends on these harmonic signals is given by theoretical derivation. Base on it, minimum theoretical detection limit for methane is calculated to be about 87×10 -9 and the minimum measurable path-integrated concentration is estimated to be about 420×10 -9 by experimentation with real situation in laboratory at 2ν 3-band R(6) line of methane.