Bioseparation is a major bottleneck in the manufacture of biopharmaceuticals such as proteins. Crystallisation is a cost-effective, rapid, and robust alternative technology to conventional chromatography steps in downstream separation processes. This thesis aims to investigate protein crystallisation as a feasible approach to separate proteins from a mixture for bioseparation. In this work, lysozyme- thaumatin mixture is used as the model binary protein mixture. Both µL-scale hanging-drop vapour-diffusion (HDVD) and mL-scale batch crystallisation methods were employed. We report an experimental evidence of direct selective protein crystallisation from a binary protein mixture solution where both proteins are supersaturated and crystallisable under an identical crystallisation condition. Results from both methods showed that protein impurity, even at low concentration level, would delay target protein crystallisation with an extended induction time. When silica particles were introduced as nanonucleants to facilitate crystallisation, target protein crystallisation was significantly improved with much shorter induction time. It was also indicated that the effectiveness of silica on crystallisation depended on the type of silica particle, silica loading amount, and impurity concentration. This study also revealed the critical role of agitation in obtaining consistent and reproducible results when moving from preliminary qualitative screenings using HDVD method to quantitative batch crystallisation experiments. Apart from improving reproducibility of crystallisation experiments, agitation also had impacts on both crystallisation rate, yield, and crystal size. In conclusion, this work demonstrates that protein crystallisation is a feasible and scalable methodology to separate a target protein from a complex mixture environment.