The pack boronizing experiment was carried out on TC4 titanium alloy surface with a method of solid powder boronizing. By means of XRD, SEM and EDS, phase composition, microstructure and morphology of the TC4 titanium alloy after boronizing were investigated. Diffusion behavior of elements in boronizing process was discussed. Results show that after holding at 1000, 1050 and 1100 oC for 5 h and 20 h, the thickness of diffusion layer ranges from 0.8 μm to 15 μm. The outer surface is TiB2 and the inner surface is TiB whiskers. XRD patterns reveal TiB-TiB2 biphasic B-Ti compounds layer is formed after boronizing of TC4 alloy and the peak position of TiB2 and TiB increases with the temperature rising. Furthermore, EDS shows, after having been absorbed by TC4, surface B atoms react with Ti of the matrix resulting in the reduction of Ti in transition region. At the same time, Al and V elements begin to diffuse into the matrix and enrich in the near-interface region. Microhardness of the diffusion layer is of gradient distribution; the hardness value of TiB2 to TiB whiskers ranges from HV0.01 22 000 MPa to HV0.01 11 000 MPa and the hardness value in the transition region is higher than that in the matrix.