  Let $(Z_n)_{n\geq0}$ be a supercritical Galton-Watson process. The
Lotka-Nagaev estimator $Z_{n+1}/Z_n$ is a common estimator for the offspring
mean.In this paper, we establish some Cram\'{e}r moderate deviation results for
the Lotka-Nagaev estimator via a martingale method. Applications to
construction of confidence intervals are also given.
