  Recently, Shen et al. (2005) studied the contributions of curvature effect of
fireballs to the spectral lag and showed that the observed lags could be
accounted for by the effect. Here we check their results by performing a more
precise calculation with both the formulas presented in Shen et al. (2005) and
Qin et al. (2004). Several other aspects which were not considered in Shen et
al. (2005) are investigated. We find that in the case of ultra-relativistic
motions, both formulas are identical as long as the whole fireball surface is
concerned. In our analysis, the previous conclusion that the detected spectral
lags could be accounted for by the curvature effect is confirmed, while the
conclusion that the lag has no dependence on the radius of fireballs is not
true. We find that introducing extreme physical parameters is not the only
outlet to explain those observed large lags. Even for the larger lags
($\sim5s$) (see Norris et al. 2005), a wider local pulse ($\Delta
t_{\theta,FWHM} = 10^{7}s$) could account for it. Some conclusions not
presented in Shen et al. (2005) or those modified in our analysis are listed
below: a) lag $\propto \Gamma^{-\epsilon}$ with $\epsilon>$ 2; b) lag is
proportional to the local pulse width and the $FWHM$ of the observed light
curves; c) a large lag requires a large $\alpha_0$ and a small $\beta_0$ as
well as a large $E_{0p}$; d) when the rest frame spectrum varies with time, the
lag would become larger; e) lag decreases with the increasing of $ R_c$; f) lag
$\propto$ E within the certain energy range for a given Lorentz factor; g) lag
is proportional to the opening angle of uniform jets when $\theta_j <
0.6\Gamma^{-1}$.
