A set of gas sensor based on quantum cascade laser (QCL) for ambient nitrous oxide (N2 O) monitoring is introduced. QCL is utilized in the center wavelength of 1274 cm-1, combining with multi-pass absorption cell. By taking the advantages of long pulsed QCL which produces a complete scan of molecular absorption lines of gases, high sensitive measurement of N2 O can be realized. Integrated software provides data analysis according to HITRAN04 database, without calibration gas. A detection limit of 13 ppb is obtained by analyzing the standard deviation and the detection of N2 O in the atmosphere is realized.