Polarized parton distribution functions are determined by using world data
from the longitudinally polarized deep inelastic scattering experiments. A
new parametrization of the parton distribution functions is adopted by taki
ng into account the positivity and the counting rule. From the fit to the a
symmetry data A(1), the polarized distribution functions of u and d valence
quarks, sea quarks, and gluons are obtained. The results indicate that the
quark spin content is Delta Sigma = 0.20 and 0.05 in the leading order (LO
) and the next-to-leading-order (NLO) <(MS)over bar> scheme, respectively.
However, if the x dependence of the sea-quark distribution is fixed at smal
l x by ''perturbative QCD'' and Regge theory, it becomes Delta Sigma = 0.24
-0.28 in the NLO. The small-x behavior cannot be uniquely determined by the
existing data, which indicates the importance of future experiments. From
our analysis, we propose one set of LO distributions and two sets of NLO on
es as the longitudinally polarized parton distribution functions.