The branching ratio of the isomeric. decay of Al-24m has been measured to be 69.6(7)% which is much smaller than the previously accepted value of 82.5(30)%. As a result, the branching ratio to the Mg-24 ground state increases up to 24.1(7)% assuming that the other beta-decay branching ratios are the previously accepted values. The half-life of Al-24m was also precisely determined to be 130.9(13) ms. The B(GT) value from the ground state of Mg-24 to the Al-24m of 0.0577(16) deduced from the beta decay is now in good agreement with that deduced from charge-exchange reactions.