  Recently PAMELA released their first results on the positron and antiproton
ratios. Stimulated by the new data, we studied the cosmic ray propagation
models and calculated the secondary positron and antiproton spectra. The low
energy positron ratio can be consistent with data in the convection propagation
model. Above $\sim 10$ GeV PAMELA data shows a clear excess on the positron
ratio. However, the secondary antiproton is roughly consistent with data. The
positron excess may be a direct evidence of dark matter annihilation or decay.
We compare the positron and anti-proton spectra with data by assuming dark
matter annihilates or decays into different final states. The PAMELA data
actually excludes quark pairs being the main final states, disfavors gauge
boson final states. Only in the case of leptonic final states the positron and
anti-proton spectra can be explained simultaneously. We also compare the
decaying and annihilating dark matter scenarios to account for the PAMELA
results and prefer to the decaying dark matter. Finally we consider a decaying
neutralino dark matter model in the frame of supersymmetry with R-parity
violation. The PAMELA data is well fitted with neutralino mass $600\sim 2000$
GeV and life time $\sim 10^{26}$ seconds. We also demonstrate that neutralino
with mass around 2TeV can fit PAMELA and ATIC data simultaneously.
