The important factors in sensors are sensitivity, selectivity, and response time. Oxide semiconductors are high sensitivity, fast response and the advantage of miniaturization. Zn-doped $SnO_2$ materials have been synthesized in order to improve the selectivity of the sensor. ZnO/$SnO_2$ crystals were prepared by a simple hydrothermal process and ultrasound pretreated hydrothermal process. ZnO/$SnO_2$ urchins were fabricated in the precursor solution with [$Zn^{2+}$]:[$Sn^{4+}$] ratio of 1:5 and rod structures were fabricated ratio of 1:1 and 1:3. Surface area ratio was increased by increasing the ratio of [$Sn^{4+}$]. The sensitivity of sensors were highest at the [$Zn^{2+}$]:[$Sn^{4+}$] ratio of 1:5 in ethanol, acetaldehyde, toluene, and nitric oxide.