The struvite crystallization process can recover struvite crystals as a valuable slow-release fertilizer from the side stream of wastewater treatment plants (WWTPs). The purpose of this study is to demonstrate the crystal growth characteristics and determine the appropriate recovery criteria for a struvite crystallization pilot plant. A pilot plant (8.6 m3/d) was designed with a feeding system of MgO (magnesium oxide), a pH controller, and a hydrocyclone for recovering struvite; the plant was operated for 42 hours at a pH range of 8.25–8.5. The removal efficiencies for PO4-P and NH4-N were 82.5–90.7% and 13.4–22.9%, respectively. The struvite recovered from the hydrocyclone was sifted using standard sieves and analyzed by SEM and XRD. The dry weight fraction of the precipitate in the 300–600 μm range increased gradually from 7% to 74% in 18 hours. The XRD analysis revealed that the crystalline structure of the precipitate in the 150–600 μm range indicates struvite without any peaks of MgO, Mg(OH)2, and MgCO3. This indicates that the critical conditions for recovering struvite from the side-stream of WWTPs are an operation period of 18 hours and a crystal size greater than 300 μm.