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 . Consequently , we obtain the total masses of the Orion A and B clouds to be ( 80.6 \pm 7.5 \pm 4.8 ) \times 10 ^ { 3 } M _ { \odot } and ( 39.5 \pm 5.2 \pm 2.6 ) \times 10 ^ { 3 } M _ { \odot } , respectively , for the distance to the clouds of 400 \mbox { $ \mathrm { pc } $ } and the Galactic CR spectrum predicted by GALPROP on the local observations of CRs . The structure of molecular clouds has been extensively studied by radio telescopes , especially using the line intensity of CO molecules ( W _ { \mathrm { CO } } ) and a constant conversion factor from W _ { \mathrm { CO } } to N ( \mbox { $ \mathrm { H } _ { 2 } $ } ) ( \equiv \mbox { $X _ { \mathrm { CO } } $ } ) . However , this factor is found to be significantly different for Orion A and B : 1.76 \pm 0.04 \pm 0.02 and 1.27 \pm 0.06 \pm 0.01 , respectively .