This paper examines the feasibility in air drilling of transmitting down-hole signals by using microwaves.Firstly the basic theory of microwave propagation in the drill-pipe or casing was studied,including power loss,cutoff wavelength,and dust scattering.Theoretical analysis indicates that the microwave propagation distance in a φ214mm casing can easily reach 5,000 m.When the effect of dust particles is taken into account,the propagation distance decreases to 2,000 m.We conducted both laboratory experiments and field tests in casings commonly used in oilfields.The field tests show that the effective propagation distance of microwave in the casing is about 1,300 m.The experimental results do not match well with theoretical prediction,but are acceptable.In future commercial applications,by applying multiple relay amplifiers,the microwave propagation distance could be long enough for most drilling wells.