Struvite crystallization is an interesting method to recover phosphorus (P) from wastewater. However, the obtained struvite crystals have a small size which is difficult to separate from wastewater. The continuous reactor for enlarging the size of struvite crystals was developed. Batch-scale experiments were conducted for investigation of optimum factors for enlargement of struvite crystals. The results of pH experiments showed that P recovery efficiency increased with an increase of pH values (7.6 to 10), while the size of struvite crystals decreased. The results of the Mg:P ratios found that the maximum P recovery efficiency occurred at the maximum ratio of Mg:P. The sizes of struvite crystals were not significantly different. For the variation of temperature values, the results showed that P recovery efficiency and crystal sizes decreased when temperature values increased. Therefore, the optimized conditions for P recovery efficiency and size enlargement of struvite crystals for the continuous reactor were at pH 8.5 and an Mg:P ratio of 1.2:1 at 30 °C (room temperature). The treated swine wastewater and seawater were continuously fed at the bottom of the reactor. After 30 days in the continuous reactor, the size of struvite crystals increased from 125 μm to 0.83 mm (7 times).