We analyze the C4 catalog of galaxy clusters from the Sloan Digital Sky
Survey (SDSS) to investigate the axis-ratio distribution of the projected two
dimensional cluster profiles. We consider only those objects in the catalog
whose virial mass is close to 10^{14}h^{-1}M_{sun}, with member galaxies
within the scale radius 1000 kpc. The total number of such objects turns out
to be 336. We also derive a theoretical distribution by incorporating the
effect of projection onto the sky into the analytic formalism proposed
recently by Lee, Jing, & Suto. The theoretical distribution of the cluster
axis-ratios is shown to depend on the amplitude of the linear power spectrum
(sigma_8) as well as the density parameter (Omega_{m}). Finally, fitting the
observational data to the analytic distribution with Omega_{m} and sigma_{8}
as two adjustable free parameters, we find the best-fitting value of
sigma_{8}=(1.01 +/- 0.09)(Omega_{m}/0.6)^{(0.07 +/- 0.02) +0.1 Omega_{m}}$. It
is a new sigma_{8}-Omega_{m} relation, different from the previous one derived
from the local abundance of X-ray clusters. We expect that the axis-ratio
distribution of galaxy clusters, if combined with the local abundance of
clusters, may put simultaneous constraints on sigma_{8} and Omega_{m}.