We present a detailed analysis of the GeV gamma-ray emission toward the supernova remnant ( SNR ) G8.7 - 0.1 with the Large Area Telescope ( LAT ) onboard the Fermi Gamma-ray Space Telescope . An investigation of the relationship among G8.7 - 0.1 and the TeV unidentified source HESS J1804 - 216 provides us with an important clue on diffusion process of cosmic rays if particle acceleration operates in the SNR . The GeV gamma-ray emission is extended with most of the emission in positional coincidence with the SNR G8.7 - 0.1 and a lesser part located outside the western boundary of G8.7 - 0.1 . The region of the gamma-ray emission overlaps spatially-connected molecular clouds , implying a physical connection for the gamma-ray structure . The total gamma-ray spectrum measured with LAT from 200 MeV–100 GeV can be described by a broken power-law function with a break of 2.4 \pm 0.6 ( stat ) \pm 1.2 ( sys ) GeV , and photon indices of 2.10 \pm 0.06 ( stat ) \pm 0.10 ( sys ) below the break and 2.70 \pm 0.12 ( stat ) \pm 0.14 ( sys ) above the break . Given the spatial association among the gamma rays , the radio emission of G8.7 - 0.1 , and the molecular clouds , the decay of \pi ^ { 0 } s produced by particles accelerated in the SNR and hitting the molecular clouds naturally explains the GeV gamma-ray spectrum . We also find that the GeV morphology is not well represented by the TeV emission from HESS J1804 - 216 and that the spectrum in the GeV band is not consistent with the extrapolation of the TeV gamma-ray spectrum . The spectral index of the TeV emission is consistent with the particle spectral index predicted by a theory that assumes energy-dependent diffusion of particles accelerated in an SNR . We discuss the possibility that the TeV spectrum originates from the interaction of particles accelerated in G8.7 - 0.1 with molecular clouds , and we constrain the diffusion coefficient of the particles .