We study the local and global stability of thin galactic disks in the context of MOG (modified gravity). In the case of local stability, we show that at large distances from the center, galactic disks are more unstable in MOG than in Newtonian gravity. In other words, MOG predicts higher star formation rates for outer parts of disk galaxies. In order to check the global stability of self- gravitating disks, we use N-body simulations. For some simple and idealized galaxy models, we show that the bar-instability exists in MOG. In other words, as in Newtonian gravity without dark matter halo, a rotationally dominated cold disk is violently unstable to a bar-like configuration.

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