Fast frequency tuning of diode-pumped monolithic semi-nonplanar ring lasers was investigated experimentally using a piezoelectric transducer (PZT). A 0.5 mm thick PZT plate was affixed to a monolithic laser crystal with dimensions of 13 mm×14 mm×3 mm. Beat frequency between two monolithic lasers was observed to check the frequency tuning effect. When the modulation frequency was lower than 100 kHz and the frequency tuning amplitude was 5 V, a widened beat signal could be observed, which gave a PZT tuning coefficient over 1 MHz/V. If the modulation frequency was much higher, such as higher than 100 kHz, the beat signal became unstable and shifted obviously. The shape of the beat signal changed too. Thus the PZT response bandwidth was near 100 kHz for a stable laser frequency tuning.