In this work, we have designed and synthesized successfully SnO2 hierarchical structures assembled by either nanoneedles or nanosheets through hydrothermal method. We found that the existence of PVP can tailor the morphologies and control the sizes of the blocking units. Moreover, nanoneedles assembled SnO2 show higher gas response while nanosheets assembled SnO2 hierarchical structures show quicker response and recovery. Thus, our work systematically synthesized size-controllable SnO2 and investigated the relationship between the morphologies and gas-sensing properties, which provide abundant information about the development of gas-sensing materials.