OBJECTIVE: The study examines the effect of the hydro-alcoholic extract of Cinnamomum zeylanicum Blume bark on crystallization of calcium oxalate. METHODS: The antilithiatic effect of various concentrations of the hydro-alcoholic extract of C. zeylanicum was investigated at various stages of stone formation, using Cystone as a standard reference drug. The effect on calcium oxalate crystallization was evaluated by measuring the change in turbidity over time, during crystal nucleation, growth and aggregation, in the metastable solution of calcium chloride and sodium oxalate. The slope from the change in turbidity over time was measured using a spectrophotometer at 620 and 214 nm in respective tests. The inhibition rate was estimated by comparing turbidity in the presence and absence of extract. Crystals formed under experimental conditions were observed under a light microscope, and number and shape of the crystals were assessed in a randomly selected field. Phytochemical analysis and high-performance thin-layer chromatography of the extract was also carried out. RESULTS: C. zeylanicum significantly reduced crystal nucleation at concentrations of 4, 8 and 10 mg/mL (P < 0.001). The inhibition percentage of crystal growth was between 28.30% and 92.46% in the presence of C. zeylanicum extract and from 20.76% to 64.15% with various concentrations of Cystone. The maximum inhibition of crystal growth was obtained from C. zeylanicum at 2 mg/mL (92.46%). Microscopic examination revealed a reduction in the number and size of crystals. In the aggregation assay, the inhibition percentage of C. zeylanicum was between 16.27% and 100%, while Cystone was from -214.68% to 100% at different concentrations. The highest (100%) inhibition of aggregation was found at 4 mg/mL of both the test and standard drugs. CONCLUSION We found that C. zeylanicum hydro-alcoholic extract has notable inhibitory effects on various stages of crystallization, in terms of turbidity of solution, as well as the crystal size, number and morphology.