An advanced laser 2-focus velocimeter (L2F) was used for the local and instantaneous measurement of droplet size in the core region of diesel fuel spray. The measuring volume of the L2F consists of two foci, where the diameter of the focus is 2 μm and the distance between two foci is 36 μm. The droplet velocity is estimated by dividing the distance between two foci by the measured time-of-flight and the droplet size is estimated by multiplying the velocity by the measured time-of-scattering. Diesel fuel was injected into the atmosphere intermittently from a single-hole nozzle with a hole diameter of 0.2 mm. Measurement was conducted on four planes where the axial distance from the nozzle orifice were 10, 20, 30, and 40 mm. It is shown that the droplet size increases with needle valve opening and varies a little after the needle valve is opened fully. The droplet size becomes smaller to direction of injection. It is found that the droplet size distribution at each measuring position in the spray core region can be fitted to the Nukiyama-Tanasawa`s distribution in most injection period including the condition of transient needle valve opening.