Abstract: With the development of drilling technology,more borehole information is needed to know while drilling and the mud pulse telemetry now hardly meets the requirement of intelligent drilling.Mud pulse MWD can not be used when too much gas or air in drilling fluid,and therefore electro-magnetic telemetry has speedily developed as an alternative.In this paper,basic transmission modes of electro-magnetic wave are discussed,the features for three transmission methods namely solenoidal,toroidal and direct coupled are analyzed in comparison.Development history of EM MWD,application situation and recent advancements are also introduced.