Strict emissions regulations and engine performance improvement requirements impose the development and use of high-pressure multi-hole diesel injectors, which are employed in direct-injection common rail systems of diesel internal combustion engines. The fuel injection and air/fuel mixing can be improved by increasing the injection pressure higher than two thousand bar. However, high injection pressures may cause cavitation and affect the initial spray characteristics, including jet shape and spray angle. The main objective of the present work was to investigate the effect of chamber pressure and temperature on the internal nozzle flow, on the initial liquid jet breakup length and on the jet angle at the vicinity of the nozzle exit. A typical diesel six-hole injector with nozzle holes symmetrically located around the periphery of the injector tip was designed and modelled. The nozzle diameter and length were 0.2 mm and 1 mm, respectively, with a ratio of nozzle orifice length over nozzle diameter L/D equal to 5. The symmetrical one-sixth segment of the injector at the nozzle tip was assembled with a constant volume chamber with square cross-sectional area with side 1 mm and a chamber length of 5 mm. A computational unstructured mesh of around one million prism cells was generated with the STAR-CD computational fluid dynamics (CFD) code. The transient CFD simulations were performed with the STAR-CD code, utilising the Eulerian methodology and the volume of fluid (VOF) method for the two-phase flow of liquid and gas, and using the Rayleigh cavitation model. CFD simulations were performed for an injection pressure of 2245 bar of liquid n-heptane which was injected in the constant volume chamber at increasing chamber pressure and temperature. Four test cases were investigated, including atmospheric chamber pressure and temperature, atmospheric pressure and chamber temperature of 700 K, and high chamber pressure of 42 bar at 700 and 1000 K chamber temperature, respectively. For all cases, cavitation occurs at the region of upper wall of the nozzle and affects the fuel/air mixing downstream the nozzle exit. The increase of chamber temperature slightly reduces the spray cone angle and has negligible effect on jet breakup length. The spray angle increases around 50 % when the chamber pressures increases by around 40 bar, and the spray angle decreases around 10% when the chamber temperature increases by 300 K. Comparisons of jet breakup length and the spray jet angle against empirical data showed moderately good agreement.