  Magnetization measurements in the low field region have been carefully
performed on a well-shaped cylindrical and an ellipsoidal sample of
superconductor $MgCNi_3$. Data from both samples show almost the same results.
The lower critical field $H_{c1}$ and the London penetration depth $\lambda$
are thus derived. It is found that the result of normalized superfluid density
$\lambda^2(0)/\lambda^2(T)$ of $MgCNi_3$ can be well described by BCS
prediction with the expectation for an isotropic s-wave superconductivity.
