  Isotopic dependence of the Casimir force is key to probing new physics and
pushing novel technologies at the micro and nanoscale, but is largely
unexplored. In 2002, an isotope effect of 10^(-4) was estimated for metals --
orders of magnitude beyond the experimental resolution. Here, by employing the
Lifshitz theory, we reveal a significant isotope effect of over 10^(-1) for
polar dielectrics. This effect arises from the isotope-mass-induced line shift
of the zone-center optical phonons and is insensitive to the linewidth. We
perform numerical analyses on both the imaginary and real-frequency axes, and
derive analytical formulas for predicting the isotope effect.
