The strain-induced band structure change in a semiconductor can change its resistivity, known as the piezoresistive effect. If the semiconductor is also a piezoelectric material, strain-induced polarization charge can control the current transport at the metal-semiconductor contact, which is called a 'piezotronic effect'. Piezotronic effect is intertwined with piezoresistive effect in the study of present piezotronic nanowire devices. Decoupling those effects will facilitate the fundamental study on the piezotronic devices and simplify the data analysis in real applications. Here, we report a general method to separate the piezotronic and piezoresistive effects in the same nanowire, based on modified four-point measurements. Current transport characteristics of each contact was extracted and showed different responses to the strain. The piezoresistive effect was measured in zinc oxide nanowires for the first time, and the result confirmed the dominant role of piezotronic effect in the strain-induced change of transport characteristics in a piezoelectric semiconductor. This study validates the assumption made in present piezotronic devices and provides a guideline for further investigation.