In the presented paper, a combined experimental and numerical study was carried out in order to study the influence of in-nozzle flow, cavitation and tested fuel properties on the spray development and primary break up process. The internal flow of neat diesel and neat rapeseed oil biodiesel fuel in single-hole diesel injectors was studied numerically using the AVL FIRE computational program. The spray development and break-up were monitored in a pressure chamber at high pressure using a high-speed camera. An innovative method for obtaining spray length and cone angle was developed using LabVIEW software. The developed method allows us to study the influence of in-nozzle cavitation inception on the symmetry of injected fuel spray. The obtained numerical results show that the geometry of the test injector influences internal fuel flow and cavitation inception highly. Experimental results indicate that tested fuel properties` influence spray length and cone angle at high atmospheric pressure is rather small. The cavitation inside the injector nozzle influences the primary spray break-up process further and causes part of the spray, where more cavitation is present in the nozzle hole and its exit, to disintegrate. This confirms that cavitation inside the fuel injector can have a positive effect on the spray break-up and spray disintegration process.