This study investigated the effect of expanded vermiculite (EV) on the flammability properties of medium-density fiberboard (MDF), which was evaluated by limiting oxygen index (LOI) and simultaneous thermal analysis (TG-DSC). In addition, the modulus of rupture (MOR) and the modulus of elasticity (MOE) of the samples were studied. The results indicated that the addition of EV increased the LOI of MDF, while it decreased the MOR and MOE of MDF quite rapidly. The TG data showed that the fiber-charring rate of the fire retardant MDF increased sharply, more than 10 times that of untreated MDF. Moreover, with increasing of the ratio of the EV and fiber (V/F), it increased the fiber-charring rate of the MDF sharply, decreased the temperature of the maximum mass loss, and decreased the maximum mass loss rate of MDF. The DSC test results indicated that the total temperature range of the exothermic stage had extended and that the first peak in the exothermic stage decreased rapidly with increasing of V/F ratio.