  Based on our improved SU(2) slave-boson approach (Phys. Rev. B 64,
052501(2001)) to the t-J Hamiltonian, we report a scaling behavior of pseudogap
with doping and the temperature and doping dependence of spectral functions. In
addition we discuss the cause of hump and quasi-particle peak in the observed
spectral functions of high Tc cuprates. It is demonstrated that the sharpening
of the observed quasi-particle peak below Tc is attributed to the bose
condensation of holon pair. From the computed ratios of pseudogap $\Delta_0$ to
both the superconducting temperature Tc and the pseudogap temperature $T^*$ as
a function of hole doping concentration x, we find that there exists a
universal scaling of these ratios with doping, that is, the hyperbolic scaling
behavior of the former, $\frac{2 \Delta_0}{k_B T_c} \sim x^{-\alpha}$ with
$\alpha \sim 2$ and near doping independence of the latter, $\frac{2
\Delta_0}{k_B T^*} \approx 4 \sim 6$ are found.
