Colloidal CdSe/CdS nanocrystals are synthesized at low temperatures (40–70 °C) via a two-phase approach at an environmentally friendly n-heptane–water interface. Oil-soluble cadmium myristate (Cd-MA) is used as cadmium source, while thiourea and selenourea are used as sulfur and selenium sources, respectively. First, the CdSe nanocrystals are achieved at an n-heptane–water interface with a diameter ranging from 1.77 to 1.83 nm at 40 °C. Subsequently, highly luminescent CdSe/CdS core–shell nanocrystals are synthesized using poorly reactive thiourea as sulfur source at 70 °C in a two-phase system. As-prepared core–shell nanocrystals have a quantum yield (PL QY) of up to 73% relative to coumarin 6 at room temperature. Meanwhile, the influence of CdSe core size and CdS shell thickness on PL QY and PL lifetime are also investigated.