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 } .