The average chain conformations of polyoxyethylene chain in the third-order interaction approximation and the effect of third-order interaction on persistence vectors are investigated. A priori probabilities of polyoxyethylene chains in the third-order interaction approximation are also calculated

and this method may also be applied to study the average chain conformations of symmetric polymer chain with complex units and of asymmetric polymer chains. The expression of persistence vector in the third-order interaction approximation is also produced.