  The upper critical field $\mu_0H_{c2}(T_c)$ of LiFeAs single crystals has
been determined by measuring the electrical resistivity using the facilities of
pulsed magnetic field at Los Alamos. We found that $\mu_0H_{c2}(T_c)$ of LiFeAs
shows a moderate anisotropy among the layered iron-based superconductors; its
anisotropic parameter $\gamma$ monotonically decreases with decreasing
temperature and approaches $\gamma\simeq 1.5$ as $T\rightarrow 0$. The upper
critical field reaches 15T ($H\parallel c$) and 24.2T ($H\parallel ab$) at
$T=$1.4K, which value is much smaller than other iron-based high $T_c$
superconductors. The temperature dependence of $\mu_0H_{c2}(T_c)$ can be
described by the Werthamer-Helfand-Hohenberg (WHH) method, showing orbitally
and (likely) spin-paramagnetically limited upper critical field for $H\parallel
c$ and $H\parallel ab$, respectively.
