The maneuvering indices K for ship's turning ability and T for ship's course keeping ability are very important values to evaluate the maneuvering chanacteristics and the motions of a vessel. When a vessel is built in the shipyard they carry out many kinds of sea trials including Z test trials to compute the numerical values of maneuvering indices. They carry out Z test during the sea trial period but they generally make only the graphs of heading angles showing the first, second and third overshoot angle without calculating the numerical values of indices K and T. They normally do not calculate the numerical values of K and T because the calculating and processing methods are a little bit complicated ones and ship owners and the masters of the vessel generally do not require the shipyard to calculate the values and submit the results to them. For this reason, the authors studied on more simplified testing and calculating method and carried out both of the actual sea trial tests of usual Z tests and more simplified tests using the training vessel Saeyoodal of Mokpo maritime university and calculated K and T indices from both trials. The calculated values from both trials are coincided with each other very well and the results were good and satisfactory.