The orientation of HDTMA+ in the interlayer of organic pillared montmorillonites prepared at different concentrations of HDTMA+ have been studied using X-ray powder diffraction (XRD) and theoretical calculation. A series of arrangement models of HDTMA+ in the interlayer of montmorillonite have been proposed as lateral-monolayer (LM), lateral-bilayer (LB), pseudotrilayer (PT), paraf-fin-type-monolayer (PM), paraffin-type-bilayer (PB). With the increase of the concentration of HDTMA+, the arrange-ment model of HDTMA+ in the interlayer of montmorillo-nites changes as lateral-monolayer→lateral-bilayer→para- ffin-type monolayer→pseudotrilayer→paraffin-type-bilayer and the packing density of HDTMA+ in the interlayer in-creases gradually. In the intermediate stages, different mod-els may coexist. The orientation of HDTMA + in the interlayer of organic pillared montmorillonites prepared at different concentrations of HDTMA + have been studied using X-ray powder diffraction (XRD) and theoretical calculation. A series of arrangement models of HDTMA + in the interlayer of montmorillonite have been proposed as lateral -monolayer (LM), lateral-bilayer (LB), pseudotrilayer (PT), paraf- fin- type- monolayer (PM), paraffin- type- bilayer (PB). With the increase of the concentration of HDTMA +, the arrange- ment model of HDTMA + in the interlayer of montmorillo-nites changes as lateral-monolayer → lateral-bilayer → para- ffin-type monolayer → pseudotrilayer → paraffin-type-bilayer and the packing density of HDTMA + in the interlayer in-creases gradually. In the intermediate stages, different mod-els may coexist.