As a thermodynamic process, Ostwald ripening is inevitable in solid solution or liquid sol, which usually leads to inhomogeneous morphologies and poor particle size distributions. For decades, how to prevent or control Ostwald ripening has been a challenge in material preparation, especially for obtaining excellent nanomaterials. Here, we propose two important concepts, critical concentration and time window, as the keys to inhibit Ostwald ripening occurring over a fairly long time. A series of precise controls were performed in preparing NaREF₄ nanomaterials by using an automatic nanomaterial synthesizer to explore the regularity in Ostwald ripening. Our results exhibit that Oswald ripening processes could be controlled exactly, which provides an effective approach for the controllable synthesis of NaREF4 nanocrystals. By controlling Oswald ripening, we have not only achieved high-quality nanocrystals but also developed a strategy for core-shell preparations. Given the ubiquity of Oswald ripening, this method can be applied to many preparation processes.