As semiconductor devices approach the sub-10 nm technology node, the self-heating effect (SHE) induced by confined geometries (e.g., FinFETs and nanosheet FETs) has emerged as a critical bottleneck affecting both performance and reliability. This challenge has prompted extensive research efforts to develop advanced characterization methodologies to investigate this effect and its corresponding influence on the device’s reliability issues. In this paper, we propose reflection-based ultra-fast measurement techniques for the continuous monitoring of the self-heating effect in advanced MOSFETs. With this approach, the self-heating effect-induced degradation of transistor drain current and the real-time temperature change can be continuously captured using a digital phosphor oscilloscope on a nanosecond scale. The thermal time constant of 17 ns and the thermal resistance of 34,000 K/W have been extracted for the short channel transistors used in this study with the help of this new characterization method. This reflection-based method is useful for the fast extraction of the thermal time constant and thermal resistance and for the continuous monitoring of current degradation as well as the real-time temperature. Therefore, this new characterization method is beneficial for the evaluation of the self-heating effect in advanced ultra-scaled MOSFETs.