So far, a special net is used to sample marine organism such as planktons in the marine biology. It is called as a plankton-net. However it often breaks the tissue of the marine organism. So scientists of the marine biology demand a good tool to observe the marine organism instead of the plankton-net sampling. On the other hand, an underwater vehicle has been a focus of constant attention in order to observe the marine organism. The vehicle should be equipped with devices to observe the marine organism and measure the sea characteristic. So Japan Agency for Marine-Earth Science and Technology (JAMSTEC) has developed a new underwater vehicle “PICASSO”. It is small size vehicle, but it can be equipped with various observation and measurement devices. And also it can be applied to a small ship as its own support ship owing to its size. In this paper, the outline of PICASSO and its key technologies are described, and results of sea trials carried out until now are shown.