Abstract: Using the large π bond theory and the molecular orbital theory, the inorganic molecular structure of AB3 type is tentatively explored. Some problems about the molecular structure such as the geometric shape and valency electronic number which have not yet been solved up to now are cleared up. Starting form the NO3- structure, the molecular orbital model of the AB3 type, including CO32-, SO3, BF3, BC13, BBr3, etc. is investigated.