Boronated graphite specimens were prepared from a grafoil heat-treated at 3000 degrees C for 30 min. The transport properties, such as the electrical resistivity, Hall coefficient and transverse magnetoresistance, and the total magnetic susceptibility were measured for the pristine and boronated specimens. For the specimen with high boron concentration, a negative transverse magnetoresistance at liquid nitrogen temperature and weakly temperature-dependent resistivity were observed. Assuming that the negative magnetoresistance is due to a three-dimensional weak localization, a calculation was carried out for the temperature dependence of the resistivity and transverse magnetoresistance at low temperatures in terms of Slonczewski-Weiss-McClure band model and a weak localization theory obtained by extending Kawabata's theory. The experimental finding agreed well with that the calculation predicted.