Junctionless nanotube field effect transistors (JLNT-FET) and inversion-mode nanotube field effect transistors (IMNT-FET) cause widespread attention for their better driving capability and excellent suppression of short channel effect (SCE), and self-heating effect (SHE) is widely studied as a key issue affecting their electrical and thermal performance. In this paper, the effects of conduction mechanisms on the electro-thermal characteristics in JLNT-FET and IMNT-FET were comparatively investigated by analyzing the effects of ambient temperature (TA), thermal contact resistance (Rtc), and spacer length (LS) on device parameters such as maximum lattice temperature (TLmax), maximum carrier temperature (TCmax), drain current (IDS), and gate leakage current (IG) of the bulk-conducting JLNT-FET and surface-conducting IMNT-FET based on TCAD numerical simulator. The results show that higher TA, larger Rtc, and smaller LS exacerbate phonon scattering which leads to severe SHE. Meanwhile, due to the difference of conduction mechanisms, the body conduction is less affected by interface scattering and phonon scattering, and JLNT-FET has better electrothermal characteristics.