An analytical molecular mechanics model is developed to obtain the size-dependent elastic properties of crystalline polyethylene. An effective “stick-spiral” model is adopted in the polymer chain. Explicit equations are derived from the Lennard-Jones potential function for the van der Waals force between any two polymer chains. By using the derived formulas, the nine size-dependent elastic constants are investigated systematically. The present analytical results are in reasonable agreement with those from present united-atom molecular dynamics simulations. The established analytical model provides an efficient route for mechanical characterization of crystalline polymers and related materials toward nanoelectromechanical applications.