This paper reports on the design of a microsystem-based thermal gas viscometer used for the determination of the quality of natural gas and its optimization through modification of its two main components: the hotplate and the capillary tube. Features, such as ultra-low power consumption (<100 mW), fast response (<30 s), high accuracy (>95%), robustness, and cost-effective mass production are the key outcome from this optimization. Two different technologies were investigated for the integrated hotplate: thick film technology using screen printing on a ceramic substrate and silicon technology based on thin films’ deposition and silicon micromachining. The sensors have been evaluated both numerically and experimentally for gas mixtures composed of nitrogen, methane, and ethane. The sensor with silicon hotplate exhibited superior performances in terms of power consumption, thermal time constant, and reproducibility of manufacturing process. The impact of the geometry of the hotplate on the performance of the sensor was investigated and the optimum design identified. The electro-thermo-mechanical tests confirmed its long-term stability. Finally, the reduction of the response time of the sensor was evaluated by modifying the diameter of its capillary tube. The experimental results showed that by increasing the diameter of capillary tube from 20 to $30~\mu \text{m}$ , the response time of the sensor decreased from 60 to 25 s.