  The curvature effect may be responsible for the steep decay phase observed in
gamma-ray bursts. For testing the curvature effect with observations, the zero
time point $t_0$ adopted to plot observer time and flux on a logarithmic scale
should be appropriately selected. In practice, however, the true $t_0$ cannot
be directly constrained from the data. Then, we move $t_0$ to a certain time in
the steep decay phase, which can be easily identified. In this situation, we
derive an analytical formula to describe the flux evolution of the steep decay
phase. The analytical formula is read as $F_\nu\propto (1+\tilde t_{\rm
obs}/{\tilde t_c})^{-\alpha}$ with $\alpha(\tilde{t}_{\rm
obs})=2+{\int_{0}^{\log (1+\tilde{t}_{\rm obs}/{\tilde{t}_c})}
{\beta(\tau)d[\log(1+\tau/{\tilde{t}_c})]}}/{\log (1 + {\tilde t}_{\rm
obs}/{{\tilde t}_c})}$, where $F_\nu$ is the flux observed at frequency $\nu$,
$\tilde t_{\rm obs}$ is the observer time by setting zero time point $t_0$ at a
certain time in the steep decay phase, $\beta$ is the spectral index estimated
around $\nu$, and ${\tilde t}_c$ is the decay timescale of the phase with
$\tilde{t}_{\rm obs}{\geqslant}0$. We test the analytical formula with the data
from numerical calculations. It is found that the analytical formula presents a
well estimation about the evolution of flux shaped by the curvature effect. Our
analytical formula can be used to confront the curvature effect with
observations and estimate the decay timescale of the steep decay phase.
