  Cosmological measurements suggest that our universe contains a dark energy
component. In order to study the dark energy evolution, we constrain a
parameterized dark energy equation of state $w(z)=w_0 + w_1 \frac{z}{1+z}$
using the recent observational datasets: 157 Gold type Ia supernovae and the
newly released 182 Gold type Ia supernovae by maximum likelihood method. It is
found that the best fit $w(z)$ crosses -1 in the past and the present best fit
value of $w(0)<-1$ obtained from 157 Gold type Ia supernovae. The crossing of
-1 is not realized and $w_0=-1$ is not ruled out in $1\sigma$ confidence level
for the 182 Gold type Ia supernovae. We also find that the range of parameter
$w_0$ is wide even in $1\sigma$ confidence level and the best fit $w(z)$ is
sensitive to the prior of $\Omega_m$.
