We give a Cramér moderate deviation expansion for martingales with differences having finite conditional moments of order 2 + ρ, ρ ∈ (0, 1], and finite one-sided conditional exponential moments. The upper bound of the range of validity and the remainder of our expansion are both optimal. Consequently, it leads to a "half-side" moderate deviation principle for martingales. Moreover, applications to quantile coupling inequality, β-mixing and ψ-mixing sequences are discussed.Keywords Martingales · Cramér moderate deviations · quantile coupling inequality · β-mixing sequences · ψ-mixing sequencesis the standard normal distribution function. Cramér type moderate deviations for sums of independent r.v.s have been obtained by many authors. See, for instance, Feller [15], Petrov [20], Sakhanenko [25] and [12]. We refer to the monographs of Petrov [21], Saulis and Statulevičius [26] and the references therein.