A symmetry criterion about chain folding has been proposed based on the symmetry of macromolecular chains and their space groups. It indicates that a regular adjacent re-entry fold of chains can take place

only if the symmetry of the chain remains unchanged after folding. This is a necessary condition. According to this criterion we can predict the possibility of chain folding with adjacent re-entry and possible chain folding planes. For example

it can be expected from our criterion that the folding planes must be parallel to the (010)or the (110) planes for isotactic polypropylene and the folding planes must be parallel to the (200) or the (110) planes for polyethylene.These are in agreement with reported erperimental result.