The paper presents the problem of crystallization of hydrated magnesium ammonium phosphate (struvite), a chemical formula MgNH4PO4 ∙ 6H2O often formed in the line of wastewater treatment plant sludge. Struvite crystallization tests were performed on a laboratory scale, assuming three independent variables which were pH, concentration of phosphate ions PO43- and the concentration of ammonium ion NH4+. The size of a search was required concentration of magnesium ions Mg2+, which occurs when struvite precipitation. It was found that the crystallization of struvite occurs in a wide range of concentrations of phosphate and ammonium ions, but most depend on the reaction. In the analyzed conditions, struvite is not precipitated at pH 7.0 from 7.5 and precipitated an increasing range of concentrations of individual components.