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In this work, magnesium borate (Mg2B2O5) and aluminum borate (Al18B4O33) whiskers with a high aspect ratio were successfully synthesized using a molten salt method. The phase transformation and morphology involution of the whiskers were studied by XRD, IR-FT, TG-DSC, and SEM-EDS with different boron/metal ratios and reaction temperatures. The results show a rich boron environment and appropriate reaction temperature to be the crucial factors with respect to the composition and morphology of the borate whiskers. Mg2B2O5 whiskers with a diameter of several hundred nanometers and a 10–60 length–diameter ratio were obtained when the B/Mg ratio was 1.5:1 and the reaction temperature was 800°C. For Al18B4O33 whiskers, a synthesis temperature of 1000°C and a B/Al ratio of 3:9 were found to be optimal with respect to the whisker morphology. A liquid-solid (LS) mechanism is proposed as the growth mechanism of the whiskers on the basis of the experimental phenomena.