Thirteen mud volcanoes are identified from the multibeam bathymetric data in the gas hydrate potential area of the upper Kaoping Slope, off southwest Taiwan. The heights of the mud volcanoes range from 65 m to 345 m and the size of their bases range from 680 m to 4,100 m in diameter. The slopes of the mud volcanoes are very steep (from 5.3° to 13.6°), suggesting that the mud volcanoes are fed by high-viscosity flows. ROV observations reveal active eruptions of the mud volcanoes MV1, MV5 and MV12, and two modes of eruption (explosive and effusive eruptions) were identified. The MV1 and MV5 are characterized by explosive eruptions and the eruptive cycles are about 3∼5 and 5∼8 minutes, respectively. The MV12 shows an effusive eruption, characterized by the continuous outpouring of mud together with gas plume. The gas plumes were found on tops of mud volcanoes MV1, MV4, MV5, MV10 and MV12, as indicated by the high-frequency sonar (EK500, EK60 sonars and multibeam echo sounder) images. The mud flows on the flank of six mud volcanoes (MV1, MV3, MV5, MV6, MV9 and MV10), illustrated by the high backscatter intensity bands from the sidescan sonar images. As evidenced by the results of investigation, the mud volcanoes in the upper Kaoping Slope off SW Taiwan are very active.