When hypersonic vehicle flies in the atmosphere at a high altitude with a high speed, plasma sheath is generated around the vehicle, and thus attenuating the electromagnetic wave signals and even interrupting the communication. Therefore the control, guidance, and navigation of hypersonic vehicle can be affected seriously by the plasma sheath. It is necessary to study this problem in reasonable ground experiment. The inductively coupled plasma (ICP) wind tunnel is an ideal equipment for studying electromagnetic transmission characteristics in plasma because it can produce uncontaminated plasma and the electrode cannot be ablated in the process of plasma production. We carry out the experiment in ICP wind tunnel. A thin slice of plasma jet is generated by a rectangular nozzle with an outlet size of m 50 mm250 mm. Plasma jets with different parameters are obtained by adjusting the operating power and inlet flow of the wind tunnel. Four kinds of states are provided with the electron densities of 7.01010, 5.01011, 3.51012 and 1.01013/cm3, and the collision frequencies of 1.5109, 1.6109, 2.0109 and 9.0109 Hz, respectively. The amplitude attenuations and phase changes of the electromagnetic waves are measured with microwave diagnostics system consisting of a vector network analyzer and high gain antennas. And electron density and collision frequency of plasma are obtained according to the transmission characteristics of electromagnetic waves in plasma. The attenuations of the electromagnetic wave in plasmas of different states are measured via microwave transmission system which is composed of a vector network analyzer and pairs of horn antennas covering a frequency range of 2.6-40 GHz. The results show that both the amplitude attenuation and attenuation band increase with the increase of electron density. The classical theory and thin layer theory are used to simulate the transmission attenuation. The results are compared with the experimental ones. The results in this paper provide basic data for further theoretical and numerical study of electromagnetic wave transmission characteristics in plasma.