  The absolute branching fractions for the decays $D^0 \to K^-e ^+\nu_e$ and
$D^0 \to \pi^-e^+\nu_e$ are determined using $7584\pm 198 \pm 341$ singly
tagged $\bar D^0$ sample from the data collected around 3.773 GeV with the
BES-II detector at the BEPC. In the system recoiling against the singly tagged
$\bar D^0$ meson, $104.0\pm 10.9$ events for $D^0 \to K^-e ^+\nu_e$ and $9.0
\pm 3.6$ events for $D^0 \to \pi^-e^+\nu_e$ decays are observed. Those yield
the absolute branching fractions to be $BF(D^0 \to K^-e^+\nu_e)=(3.82 \pm
0.40\pm 0.27)%$ and $BF(D^0 \to \pi^-e^+\nu_e)=(0.33 \pm 0.13\pm 0.03)%$. The
vector form factors are determined to be $|f^K_+(0)| = 0.78 \pm 0.04 \pm 0.03$
and $|f^{\pi}_+(0)| = 0.73 \pm 0.14 \pm 0.06$. The ratio of the two form
factors is measured to be $|f^{\pi}_+(0)/f^K_+(0)|= 0.93 \pm 0.19 \pm 0.07$.
