We have studied the formation of L-leucine nanoparticles under various conditions using an aerosol flow reactor method. Temperatures and L-leucine concentrations for the experiments were selected to vary the saturation conditions for L-leucine in the reactor. In the two extreme cases, L-leucine is either in (i) the condensed phase (110°C) or completely in (ii) the vapour phase (200°C) for all concentrations; (iii) at the intermediate temperature (150°C), the extent of evaporation of L-leucine depends notably on the concentration, and thus partial evaporation and production of residual particles are expected. The size distribution of particles and the particle morphology varied according to formation mechanism with the geometric mean diameter of the particles between 30nm and 210 nm. Hollow, spherical particles were obtained with the droplet-to-particle method without vaporisation of L-leucine; whereas leafy-looking particles were produced by homogeneous nucleation of supersaturated L-leucine vapour and subsequent growth by heterogeneous vapour deposition.