Big-Bang nucleosynthesis (BBN) is an important cosmological process to test not only the fundamental theory of particles and nuclei but also the cosmological theory of accelerating universal expansion and dark matter and dark energy. We first discuss several fundamental questions and discrepancies of cosmological parameters which present different Omega_b values predicted from BBN, cosmic microwave background (CMB) anisotropies, etc. We then critically discuss uncertaities in particle physics and nuclear physics which may resolve partly the cosmological discrepancies. They are on the newly measured neutron life, poorly known or unmeasured nuclear reaction cross sections, and broken neutrino-lepton symmetry. We second discuss slightly astronomical aspect which removes a tention, where the supernova (SN) neutrino-procees plays an essential role in constraining chemical evolutin of the light elements from the Big-Bang. We third propose a model of cold dark matter in brane world cosmology, and discuss how the BBN constraints allow this model, satisfying the other observational constraints from CMB, SNIa magnitude-redshift relation, and galaxy M/L ratios.