An oil-retaining structure for fan comprises an oily bearing with a central axis hole for pivoting a fan shaft. An oil-collecting recess trench is formed on an inside wall of the central axis hole. There is an acute angle oil-guiding ring edge correspondingly formed at one side of the oil-collecting recess trench. When the fan rotates, lubricant inside the oily bearing is upwardly sucked and blocked by the acute angle oil-guiding ring edge, and returns to the oil-collecting recess trench so as to construct an internal-recycle oil-retaining system.