Mineralogical transformations and microstructures during the firing alluvial clays from Monoun plain were studied. The firing of clays was carried out in the temperature range from 800 to 1150 o C. Minerals transformation was investigated by X-ray diffraction, polarize stereomicroscopy and scanning electron microscopy (SEM). In the raw materials, typical assemblages with quartz, kaolinite, goethite, anatase, hematite, gibbsite and feldspar were observed. Many neoformed mineral phases were identified with the reaction products including mullite, cristobalite, spinel, amorphous phase which occurred in the high temperature (1050-1150 o C) in the fired bricks. The quartz phenocrist with some microlites oxides embedded in few mesostasis glass have been illustrated at 950 o C by stereomicroscopy analysis. At 1150 o C, the vitreous matrix has embedded phenocrist of quartz and other relic minerals. The best temperature for fired alluvial clay brick was discovered between 1050-1150 o C.