Carbon nanotubes (CNTs) are promising and useful materials for several applications due to their good thermal, electrical, optical and mechanical properties. Many studies have been performed in the past decade to explore and develop devices which could leverage the excellent properties of individual CNTs and their two- and three-dimensional (2D and 3D) networks. In particular, CNT network thin film transistors (CN-TFTs) have been explored for a wide range of applications such as flexible displays, sensors, antennas, etc. CNT networks are typically supported on thermally insulating substrates such as glass or plastics, which have very low thermal conductivity and where the excessive self-heating in CN-TFTs under high field operations can lead to the breakdown of these devices. In addition, the variations in the channel geometry and network morphology are very likely to influence the reliability and breakdown behavior of CN-TFTs as well. The variation in the breakdown behavior for a given TFT geometry can lead to instability and/or unreliability during the operation of CN-TFTs. Thus, it is very important to understand how the geometrical parameters affect the high-field operation of the CN-TFT in order to optimize the device design for reliable and uniform behavior. We apply both experimental and computational methods to understand the breakdown behavior and thermal reliability of CN-TFTs. We examine the breakdown characteristics, such as peak power and the breakdown voltage of CN-TFTs, to find their relation with the aforementioned geometrical parameters. The analysis on breakdown behavior of CN-TFTs can provide useful insights and design guidelines for highly reliable, uniform and improved thermal/electrical performance of these devices.