The three components of the geomagnetic field were measured at sea by STCM (Shipboard Three Component Magnetometer) with a relative accuracy of 50 + or - 25 nT. The STCM was controlled by a microcomputer, and the three geomagnetic components were calculated by reducing the magnetic field due to the ship's remanent and induced magnetic moments, the only assumption being that the magnetic field is linearly related to the magnetic moments. Although STCM has no particular magnetic compensation device, it can be set at any place on board of any kind of ship. In the KH82-5 cruise, the three component measurements were conducted along latitude 42 degrees N north of the Mendocino fracture zone where there are well-defined magnetic anomaly lineations trending north and south. STCM measured the north component anomalies with very small amplitudes compared to the east and downward component anomalies, which shows that lineations trend north and south. The three measured component anomalies are almost identical to the anomalies calculated from the well-established time scale; thus, STCM measures the three components correctly.The vector geomagnetic measurements are more useful for analysis of marine geomagnetic anomalies than are the total-intensity measurements. The DELP-WAKASHIO 84/1 cruises provide good examples for examination of the linearity of magnetic anomalies on a single track by using the Fourier transform of the horizontal and vertical component anomalies.