Abstract: Based on screw theory and spatial structure analysis, the structural formulae of all order mechanisms were deduced and the classification of spatial bar groups was made. The number of general constrain was introduced to determine the order of the bar group and the number of the components was used to determine the grade of the bar group. All the forms of spatial bar group with less than three components and no general constraint were synthesized. By the classification, the grade of spatial bar group could be decided. It gives a new theoretical foundation for space mechanism creative design.