The conditions of crystallization of micaceous iron oxide （MIO） were studied through the synthesis of MIO from precursor Fe（OH）3 formed during oxidization of FeSO4·7H2O with H2O2. The experimental results showed that the concentration of NaOH as a hydrothermal medium had a strong effect on the formation,morphology,color and luster of crystal. The red iron oxide,yellow iron oxide and MIO with various types of crystal could be produced in the range of 0.019-14 mol·L -1 of NaOH concentration with the reaction temperature equating to 433 K or above. The color of MIO crystal produced varied from reddish brown,brown to steel grey with increasing NaOH concentration.The reaction temperature also affected the formation,morphology,color and luster of crystal. There was a great difference in conversion time from precursor Fe（OH）3 to MIO because of the changes of both NaOH concentration and reaction temperature. It was an effective method to age the crystal for a certain length of time to get more thinner MIO. The preferred technology parameters for the crystallization of MIO were as follows:a reaction temperature of 443-453 K,a NaOH concentration 12-14 mol·L -1 and a reaction time of 250 min.
