Public interest, no doubt, is the most important goal and concept in Public Administration. However, its definition is still quite vague now. So, it is clearly a problematic concept. Most people in modern society confirm public administrators are the number one definers and executers for public interest. Three different approaches, content approach, process approach and subjective approach, are mentioned in this article to help public administrators to define public interest. Content approach emphasizes to ascertain the real substance of public interest. Process approach intends to set the necessary processes to define public interest. Subjective approach centre on the inner world of public administrators when they define public interest. The relationships of these three approaches are mutually supportive in stead of mutually exclusive.Keywords:public administrators , public interest