Lift-off length is the distance from injector to the location where flame stabilized under quiescent conditions on a high injection pressure direct injection (DI) diesel spray. In this paper we simultaneously used high-speed (40 kHz) schlieren and time-averaged OH chemiluminescence imaging technique to research the flame lift-off locations on a DI diesel spray in an optically accessible and constant-volume combustion vessel. Visualized temporal progression of the diesel spray that acquired from high-speed schlieren imaging system, enabled us to observe the instantaneous spray structure details of the fuel injection progress. Meanwhile the OH chemiluminescence image used to determine the quiescent lift-off length were gained from a gated, intensified CCD video camera with different delay and width settings. Experiments were conducted under various ambient temperature, ambient gas density, injection pressure and oxygen concentration. From the schlieren images of the injecting procedure, along the axis of the spray from the injector, a distinct expansion located in the downstream, after which the edge of the spray changed to transparent gradually and the refractive index gradients started to diminish. This point was defined as the flame location of high-speed schlieren and the length from it to the injector was measured. It is found that the schlieren imaging lift-off length fluctuated within a certain range during the injection process, and the average lift-off length of quiescent conditions was also calculated. The results of the high-speed schlieren coincided well with the lift-off length results acquired from the CCD results, indicating high-speed schlieren imaging technique can be used to measure the flame lift-off length on DI diesel spray.