Spray autoignition is an essential issue in many combustors, e.g., diesel engine and aeroengine. Three-dimensional direct numerical simulation coupled with complex chemistry is adopted to investigate n-dodecane spray autoignition at high temperature. The ignition processes are analyzed in both physical space and mixture fraction space. As time goes on, the fraction of premixed combustion increases, and in a later stage the heat release is mainly contributed by the premixed combustion. The effects of initial droplet temperature are studied. It is found that lower initial droplet temperature induces longer evaporation lifetime and larger temperature depression; thus ignition occurs later. The performances of typical heat release indicators are also quantitatively evaluated. The relative contributions to heat release of elementary reactions are examined to explain the complex correlations between [OH]center dot[CH2O] and heat release rate.