$\beta $ -Ga2O3 is an attractive material for high-voltage applications and has the potential for monolithically integrated RF devices. A combination of Raman nano-particle thermometry measurement and thermal simulation has been used to measure the peak channel temperature due to self-heating in $\beta $ -Ga2O3 MOSFETs. The peak channel thermal resistance measured at the gate surface in the device center was $88~mm\,\! \cdot \, K/W$ . This value is higher than what has been previously reported using electrical methods, which determine an average temperature over the whole device area. Experimentally validated thermal simulations have been used to propose possible thermal management mitigation approaches.