Employing a grating-external-cavity diode laser (ECDL) as laser source, the modulation transfer spectrum (MTS) of cesium D 2 line (6 2S 1/2 F=4→6 2P 3/2 F′=3,4,5) is obtained. Instead of using a electro-optical modulator (EOM), a directly frequency modulation to an acousto-optical modulator (AOM) in MTS device is adopted, which more or less simplifies the spectroscopic device. Under different modulation frequencies the character of MTS is investigated experimentally.