Hierarchical three dimensional MoS2 nanostructures composed of small sheet-like nanostructures are successfully prepared using a simple two-step hydrothermal process with sodium dodecyl sulfate (SDS), as a surface modifier. The as-prepared MoS2 nanospheres are characterized by XRD, BET, XPS, EDS, SEM and HRTEM. The gas-sensing properties of as-prepared MoS2 nanostructures are tested at 175-230 degrees C temperatures for 50-500 ppm concentration range, using an efficient ceramic-tube sensor system via an intelligent gas testing system. A high response and selectivity towards carbon monoxide than other gases is observed. Comparative studies of MoS2 nanostructures for different concentration of carbon monoxide gas and definite sensor-stability for different gases are studied, as well. The sensor fabrication and sensing mechanism is discussed as well.