The present invention provides a diaminopyrimidine derivative or its pharmaceutically acceptable salt, a process for the preparation thereof, a pharmaceutical composition comprising the same, and a use thereof. The diaminopyrimidine derivative or its pharmaceutically acceptable salt functions as a 5-HT4 receptor agonist, and therefore can be usefully applied for preventing or treating dysfunction in gastrointestinal motility, one of the gastrointestinal diseases, such as gastroesophageal reflux disease (GERD), constipation, irritable bowel syndrome (IBS), dyspepsia, post-operative ileus, delayed gastric emptying, gastroparesis, intestinal pseudo-obstruction, drug-induced delayed transit, or diabetic gastric atony.