**Abstract**

Microwave heating is widely used to accelerate organic reactions in the chemistry field. However, the effect of microwaves on chemical reaction has not yet been well characterized at the molecular level. In this review chapter, microwave heating processes of liquid crystals and an ethanol-hexane mixed solution under microwave irradiation were experimentally and theoretically investigated using *in situ* microwave irradiation nuclear magnetic resonance (NMR) spectroscopy and molecular dynamics (MD) simulation, respectively. The temperature of the solution under microwave irradiation was estimated from a chemical shift calibrated temperature (CSC-temperature) which was determined from the temperature dependence of the 1 H chemical shift. The CSC-temperatures of CH2 and CH3 non-polar protons of ethanol reflect the bulk temperature of a solution by the thermal microwave effect. The lower CSC-temperature of the OH polar protons in ethanol and much higher CSCtemperature of H-C=N (7′) and CH3-O (α') protons of N-(4-methoxybenzyliden)-4 butylaniline with respect to the bulk temperature are attributed to the non-thermal microwave effects. According to the MD simulation under microwave irradiation, the number of hydrogen bonds increased in the ethanol-hexane mixed solution as a result of a non-thermal microwave effect. It is concluded that a coherently ordered low entropy state of polar molecules is induced by a non-thermal microwave effect. The ordered state induces molecular interaction, which may accelerate the chemical reaction rate between molecules with polar groups.

**Keywords:** microwave heating mechanism, microwave irradiation NMR spectroscopy, MD simulation, thermal microwave effect, non-thermal microwave effect
