Both the left-and right-handed circularly polarized radiations of the masers in the ultra-compact hydrogen II region G45.07+0.13, due to the transition from the OH ground state 2II 3 2 , J = 3 2 , F = 1 → 1, have been observed with the US VLBI array. using the multiplepoint fringe rate method, we have constructed the map of the maser structure with a relative position accuracy of 20 mas. With the fringe rate of the calibration source and that of the maser component with a velocity of 56.7 km s −1, taken to be the phase reference point, the absolute position of the phase reference point has been calculated as given by α (1950.0) = 19 h11 m00.46 s, δ(1950.0) = 10°45′ 43.2″. We have found that most maser components are located in the front of the comet-shaped HII region and that their distance to the center of hydrogen II region is approximately 0.4″. This agrees well with the distance from the vertex of the parabolic ionization wavefront to the young celestial body, as estimated with the bow shock model. By use of the tube-shaped model and under the assumption of fully saturated radiation, the number density of the hydrogen molecules in maser regions has been derived to be 4.5×10 8cm −3. Such high density regions are very possibly the thick clouds of neutral molecules swept up by shock waves and are located in the front side of ionization wavefront. In this maser active region a pair of Zeemansplit lines have been found and from the difference of their radial velocities a magnetic field of 3 mG has been inferred. Its direction is away from the Earth. Similar to the observation of G34.3±0.2, the diameter of the maser cluster near the compact hydrogen II region has the order of magnitude of 10 16 cm. This is 50 times larger than the thickness of a neutral gas layer possibly swept up by shock waves. It is still not clear how this can be explained.