Alloy nanocrystals (NCs) provide access and control of the parameters for nanoscale engineering because their physical and optical properties depend on size, shape, and composition. Here, alloy CdS x Se1−x NCs with different shapes were synthesised via a one-pot method using cadmium acetate, sulfur, and selenium as precursors in trioctylphosphine solution. The luminescence and shapes of NCs were characterised by fluorescence spectroscopy and transmission electron microscopy, respectively. It was found that the modification of precursor concentration resulted in NC shape variation, including branched NCs, long and short nanorods. Consequently, a series of alloy CdS x Se1−x NCs with different shape-based light emitting devices (LEDs) were fabricated and their operation characteristics were also compared. The obtained luminance and luminous efficiency showed that the control of NC shape is the key factor for the improvement of LED performance. We anticipate that this work will provide further insight into the design of CdSxSe1−x NCs-based LEDs with shape variation.