A low-frequency wavelength modulation spectroscopy is acquired with an external-cavity tunable diode laser. The wavelength modulation is achieved using the voltage tuning by scanning the piezoelectric stepper motor with rotating of the end mirror in the laser cavity. With optimum modulation frequency of 1-kHz a harmonic detection processed by a lock-in amplifier for cesium D2 line was carried out. It is found that 6f-harmonic detection is best here with the signal-to-noise voltage ratio (SNR) being 460. It is also shown that nearly overlapping lines may be better resolved by using 6f harmonic detection due to the much less modulation-broadened spectra.