Long period oscillations inside the harbor are always the major problems for the ports along Taiwan east coast. In order to investigate the resource of the oscillations, a series of field obasevations of water elevation at Hualien Port, and Yen-Liao, Gin-Jun, and Da-Wu fishery ports along Taiwan east-south coast have been conducted with long sampling time. The results show that wave motions with period longer than 100 seconds were found everywhere within that 130 km coastline in ordinary time and in typhoon periods. The resource of long period oscillations inside a harbor along Taiwan east coast are not definitely related to the natural frequencies of that harbor. Changing the harbor layout to shift the natural frequencies is not the solution of the problem, the future studies should focus on searching the method to deduct the long period wave energy inside the harbor.