Bio-separation is a key bottleneck in the manufacture of biopharmaceuticals. In this work, we report experimental evidence of direct selective protein crystallisation from a binary protein mixture solution. Lysozyme–thaumatin mixtures with a wide protein composition range (0–100 mg mL−1, respectively) were tested under the same crystallisation cocktail conditions using the hanging-drop vapour-diffusion (HDVD) crystallisation method. This work demonstrates the selectivity of crystallisation from a model binary protein mixture and four crystal occurrence domains were determined as the operation windows of selective crystallisation of the target protein: 1) an unsaturated region with no crystal formation, 2 & 3) target regions with only a single type of protein crystals (lysozyme crystals only or thaumatin crystals only) and 4) a mixture region which have a mixture of both types of protein. This study demonstrates that protein crystallisation is not only applicable to high-purity protein solutions and emphasizes the vital impacts of the presence of protein impurities in the process of target protein crystallisation. The study concludes that protein crystallisation is a feasible approach to separate a target protein from a complex mixture environment which can be achieved by manipulating the crystallisation operation conditions such as mixture composition, precipitant concentration, and operation time.