We report on a preliminary analysis of the diffuse gamma-ray observations of local giant molecular clouds Orion A and B with the Large Area Telescope onboard the Fermi Gamma-ray Space Telescope. The gamma-ray emission of the clouds is well explained
by hadronic and electromagnetic interactions between cosmic rays and nuclei in the clouds. In consequence, we obtain the total masses of the Orion A and B clouds to be (80.6 +/- 7.5 +/- 4.8) x 10^3 Msun and (39.5 +/- 5.2 +/- 2.6) x 10^3 Msun, respectively, for the distance to the clouds of 400 pc and the Galactic CR spectrum predicted by GALPROP on the local observations of CRs. The structure of molecular clouds have been extensively studied by radio telescopes, especially using the line intensity of CO molecules (WCO) and a constant conversion factor from Wco to N (H_2) (= Xco). However, this factor is found to be significantly different for Orion A and B: 1.76 +/- 0.04 +/- 0.02 and 1.27 +/- 0.06 +/- 0.01, respectively.
