One of the most commonly used boron compounds, boric acid, is produced by dissolving colemanite (2CaO&times / 3B2O3&times / 5H2O) in aqueous sulfuric acid whereby gypsum (CaSO4&times / 2H2O) is formed as a byproduct and must be separated from the main product. This process consists of two steps, dissolution of colemanite and formation of gypsum. The amount of boric acid formed depends on the first step, dissolution of colemanite. In the latter step, gypsum crystals are formed and stay in the reaction mixture to grow up to a size large enough to be filtered out of the solution. Filtration of gypsum crystals is a crucial process in boric acid production because it affects the purity and crystallization of boric acid. In this study it is aimed to investigate the effects of particle size of colemanite, stirring rate and reaction temperature on the dissolution of colemanite, gypsum formation and particle size distribution of gypsum formed in the reaction of boric acid production. Colemanite, sulfuric acid and distilled water were used as reactants for the boric acid production reaction in this study.The colemanite minerals were provided from a region of Emet, Kutahya, Turkey. Three types of colemanite minerals having different chemical composition and particle size were used. The sulfuric acid was supplied by Eti Holding A.S. Hisarcik 1 and Hisarcik 2 colemanites were crushed in a jaw crusher, ground in a hammer mill and then sieved. The sieve analysis was performed to learn the size distribution of Hisarcik 1 and Hisarcik 2 colemanite. Hisarcik 3 colemanite was brought from Emet Boric Acid Plant. The maximum diameter of the colemanite minerals was 150 &amp / #956 / m. The experiments were performed at different particle sizes of colemanite (0-150, 0-250 and 250-1000 &amp / #956 / m), temperatures (70- 90 &amp / #61616 / C) and stirring rates (350-500 rpm). The photographs of gypsum crystals were taken. The boric acid and calcium ion concentrations were determined for each experiment. Also, the solid content of the solution in the reactor were measured. The dissolution of colemanite can be followed by monitoring the boric acid concentration change in the slurry. The crystallization of gypsum from the solution can be found from the calcium ion concentration in the solution. The crystallization kinetics of calcium sulfate dihydrate was studied. The growth of the gypsum crystals were examined under the light microscope and the particle size distribution of gypsum crystals were analyzed by of the laser diffraction instrument.