The spray characteristics and inside flow of a marine diesel injector were investigated both experimentally and numerically. From the experiments, we observed that the penetration of the sprays in the early injection stage gradually increases. This phenomenon differs significantly from that of the small automobile diesel injector, in which penetration increases linearly with time. Using the momentum method to obtain injection rate measurements, we observed an injection rate spike at each injection event just after the injection began. The observed spray results show that the small portion of fuel remaining inside the nozzle from the previous injection event is ejected first, and then the main volume of fuel is ejected. Both fuels accumulate as spray droplets and gradually accelerate after the early injection stage. Numerical simulations of the injector's inside flow show that the fuel injection rate becomes saturated in needle lifts larger than 0.3 mm. Cavitation can be observed around the needle seat for smaller needle lifts. A larger cavitation area appears around the needle seat during the lifting movements than during the closing movements of the same needle lift. The difference in the injection amount from each of the five nozzle holes varies by approximately 8% because of the differences in the inside flow. As the deflection angle between the sac and nozzle hole greatly influences the flow around the entrance area and inside the nozzle holes, the discharge coefficient decreases in the nozzle holes with a larger deflection angle.