A Highly ordered CdS nanoflake array was fabricated by CVD, and its gas sensing characteristics were investigated. The sensor exhibited high response (resistance ratio) of 89% to 5 part per million (ppm) nitrogen dioxide (NO2) under green LED illumination (wavelength 500–540 nm, irradiance 21 W/m2) with excellent selectivity and little interference by humidity. Moreover, the sensor showed promising potential for operating under fluorescent lamp and natural solar light, which can be used for medical diagnosis and indoor/outdoor environment monitoring. This performance is attributed to the low band gap energy (2.4 eV) of CdS and the unique morphology of nanoflake array which can enhance both the light absorption and conductivity.