By analyzing about 33 pb−1 data sample collected at and around 3.773
GeV with the BES-II detector at the BEPC collider, we directly measure the
branching fractions for the neutral and charged D inclusive semimuonic decays
to be BF(D0→μ+X)=(6.8±1.5±0.7) and BF(D+→μ+X)=(17.6±2.7±1.8), and determine the ratio of the two branching
fractions to be BF(D0→μ+X)BF(D+→μ+X)=2.59±0.70±0.25