In this work, atomic connectivity group contribution (ACGC) method is developed for predicting critical properties of organic compounds. Herein, a new group defining method, namely atomic adjacent group (AAG) method, is proposed to describe the relationship between core atom and its adjacent atoms. For distinguishing isomers effectively, the shape factor (SF) is used to describe the effect of molecular shape on group, and atomic connectivity factors (ACF) are defined for describing the position of each group in a molecule. The external and internal verification methods were utilized during the modelling process. Compared with AAG model, ARE decreased by 6.82-42.57 % when SF was considered and, ARE decreased by 24.19-62.25 % when both SF and ACF were applied as using the ACGC method. Accordingly, SF and ACF are effective in improving the group contribution method and ACGC method is accurate in calculating the properties of organic compounds.