In this work, using the length-width-height ratios of a rectangular box in which the molecule under study is enclosed, two descriptors for the characterization of molecular shape are introduced. The molecular shape analysis of the set of molecules under study is facilitated by the introduction of a diagram where the extreme cases (i.e. cubic-box, prolate-box and oblate-box) are clearly located. The method is illustrated by the analysis of fifty molecules including normal alkanes, acenes, alkynes, oblate molecules and the eighteen octane isomers. The geometry of each molecule was optimized by AM1 semiempirical method, and van der Waals molecular surface was used to define the molecular shape contour. The length, the width, and the height of the enclosing box were extracted using VEGA ZZ software. A regular behaviour of the shape descriptors for homologous series was observed. Finally, using the present descriptors, a molecular shape similarity analysis of the octane isomer molecules was included.