ZnO with rice-like, stare-like, conical and short rod morphologies have been synthesized by a simple hydrothermal and microwave method using zinc acetate, NaOH and different templates as sucrose, galactose and hexamine. The influence of the type of template and synthesis condition on structural, optical and sensing properties was studied by X-ray diffraction, scanning electron microscopy, UV–Vis spectra, and a gas sensor characterization system. The results show the template and synthesis conditions to have significant effect on morphology, crystallite size, band gap and sensing properties. Star-like ZnO microstructures show the highest response, i.e. 40, to CO gas at 250 °C than the other sensors. Sensing mechanisms are discussed in detail.