The design, fabrication and experimental validation of two MicroElectroMechanical Systems (MEMS) Double-Ended Tuning-Fork (DETF) resonators that exhibit an intrinsic sub-350 ppm thermal stability in the temperature range [5°C-85°C] are reported. A strategy for the optimization of the design of MEMS resonators that exhibit high thermal stability and high quality factor is also provided and a good agreement with experimental data is achieved. The main advantage of the proposed strategy is that it does not require any experimental calibration of the model parameters and can be in principle applied to different kind of resonators, thus representing a powerful tool for the apriori design of thermally stable MEMS resonators with high quality factors.