A viscoelastic theory is adopted to analyze the optical effects in double-coated polymeric optical fibers subjected to thermal loadings. A closed-form solution of the microbending loss and refractive index changes obtained by using the Laplace transformation method is presented. To obtain the solution, the compatibility conditions of displacement at interfaces and Lame's formula for the stress components are used. The results are initially identical to those obtained by the elastic analysis, and then decrease gradually as time progresses. To minimize the microbending loss, the polymeric coatings were suitably selected, so that the Young's moduli, Poisson's ratios, and viscosities of the primary and secondary coatings would decrease. Furthermore, selecting suitable primary and secondary coatings with lower Young's moduli, Poisson's ratios, and viscosities reduce the changes of refractive index in the glass fiber.