Sensitivity modification of tunable ultrasonic microsensors has been investigated in terms of poling effect to the piezoelectric/ferroelectric thin film on the sensor diaphragm. The sensor has a tuning electrode besides a sensing electrode to tune the resonant frequency by introducing converse-piezoelectric stress on the diaphragm. Sensitivity change with poling and tuning voltage applications has been traced on sensors with flat and buckled diaphragms and the buckled-diaphragm sensors have shown anomalies; sensitivity decreases after increase with increasing poling voltage and a butterfly curve of sensitivity to tuning voltage reverses in a high poling voltage region. After the sensitivity investigation the buckling shape of the diaphragm has been profiled under the poling and tuning voltage applications, and the origin of the sensitivity anomalies have been found to be the total stress of the diaphragm and buckling shape of the sensing electrode part induced by the combination of the poling and tuning stress.