A selecting method of light guide plate of backlight module is described. The selecting method includes the steps of: calculating a plurality of mura indexes (MI) corresponding to a plurality of mura statuses of a plurality of first light guide plate (LGP) types, respectively; defining a plurality of film structures, wherein each of the film structures corresponds to each of mura indexes for mapping the mura indexes (MI) of the first LGP types with the film structures to construct a mapping database; and selecting one of the film structures and one of the mura indexes (MI) correspondingly from the mapping database for determining a critical dot dimension (CDD) of a second LGP type of the selected film structure. The selecting method avoids the mura, speed up the research and development procedure of the backlight module, labor cost and manufacturing cost when the LGP is assembled with the film structure.