The application of selective resonant tunneling model is extended
from d + t fusion to other light nucleus fusion reactions, such as d +
d fusion and d + 3He. In contrast to traditional formulas,
the new formula for the cross-section needs only a few parameters to
fit the experimental data in the energy range of interest. The features
of the astrophysical S-function are derived in terms of this model. The
physics of resonant tunneling is discussed.