We explore the dependence of power losses on average plastic energy densities as rays propagate along deformed polymer optical fibers (POFs). The variation of power losses in deformed POFs with different bend radii and elongations are measured and analyzed. Three-dimensional elastic-plastic finite-element models are used to calculate average plastic energy densities in deformed POFs. The results indicate that the average plastic energy density introduced in a deformed POF can be considered a key index with which to study the power loss. Based on the experimental results, a curve-fitted equation is proposed for estimating the power loss by using the average plastic energy density for various bend radii.