During the past years, the accuracy of relative positioning using differential GPS (DGPS) has been improved significantly. The present accuracy of DGPS allows us to directly estimate the differential amplitudes and Greenwich phase lags of the main semi-diurnal ocean tide loading constituents (S-2, K-2, M-2 and N-2). For this purpose a test is carried out using two GPS stations in Alaska. One station, Chi3, is located on an island in the Gulf of Alaska, while the second station, Fair, is located far away from the coastal areas. Processing hourly GPS solutions for the baseline between Fair and Chi3 during 49 days gives differential amplitudes of 23.21 mm and 4.71 mm for M-2 and N-2, respectively, while the theoretically differential amplitudes of M-2 and N-2 are 20.90 mm and 4.21 mm, respectively (using the GOT99.2 ocean tide model). The diurnal ocean tide loading constituents are not considered, because unmodeled troposphere effects increase the noise level near the diurnal frequency band and prevent us from obtaining significant results.