An atmospheric process for the synthesis of scorodite (DMSP®) has been developed, in which ferrous ions were oxidized by oxygen gas in the presence of pentavalent arsenic ions. Under optimum conditions, the synthesized scorodite (FeAsO4･2H2O) was well crystallized and the crystals were faceted; the particle sizes were as large as 15 to 20 µm. In this study, iron (II) sulfate heptahydrate (FeSO4·7H2O), arsenic (V) acid (60 mass% H3AsO4) solution, and distilled water were used to prepare the original experimental solution (700 ml), in which the contents of Fe(II) and As(V) were 55.9 g/L and 50.0 g/L, respectively. The effects of various synthesis conditions, such as the type of oxidizing gas (oxygen or air), gas injection rate, temperature, and the initial pH of the solution, on the size and shape of the scorodite particles were investigated. From the experimental results, the changes in the shape of the scorodite particles were clarified for each synthesis condition.