The application of microwave irradiation as a new kind of energy source for activation of reactions has become a popular and useful technology in organic chemistry. The high heating efficiency of microwaves brings, in many cases, marked increase of the reaction rate and dramatic reduction of the reaction time. Many studies in the past decade on organic synthesis by microwave irradiations showed that this was because of dielectric heating effects of microwaves, while some other studies suggested that non-thermal effects of microwaves could change the reaction dynamics and decrease activation energy of organic reactions. The paper reviews recent progresses in the enhancement of microwave to organic reactions and the reaction mechanisms.