We demonstrated the synthesis of Pt–Ni alloy core–shell nanoparticles (NPs) via a one-pot thermal decomposition method, optimized by variation of the concentration of cetyltrimethylammonium chloride (CTAC) and reaction time. The samples prepared without CTAC and in 30 mM CTAC at 250 °C for 180 min exhibited the formation of single Pt-rich phases between metallic phases. With increasing CTAC concentrations (60–120 mM) at a constant temperature and time (250 °C for 180 min), the products contained both Pt-rich and Pt–Ni alloy phases, consisting of a Pt-rich core with a Pt–Ni alloy shell (Pt-rich@Pt–Ni), in contrast to the single Pt-rich phases prepared at low concentrations or in the absence of CTAC. As the reaction time increased from 10 to 180 min in 60 mM CTAC at 250 °C, the Pt-rich NPs were observed to grow in the initial stage, i.e. until a critical reaction time of 60 min, with subsequent formation of the Pt–Ni alloy phase on top of the as-formed Pt-rich NPs. The morphology and structure of the as-prepared NPs were characterized using TEM, EDX and XRD.