In our work, CdS nanocrystals were simply prepared via coprecipitation method and subsequent annealing process (300, 400, 500 ◦C). X-ray powder diffraction (XRD) and the transmission electron microscope (TEM) etc. methods were used to analyze structure morphologies of the samples. The unannealed CdS nanocrystal is cubic, its phase evolution depends on annealing temperature. At 300 ◦C, its cubic phase was remained. Annealing at 400 ◦C makes small amount of hexagonal phase produced and cubic/hexagonal homojunctions were formed. Thorough phase transition to hexagonal phase was completed at 500 ◦C. The sensing properties based on the synthesized CdS nanocrystals with different phase structures were measured. The CdS-400 based sensor had the highest sensitivity to n-propanol, which was 41 at the optimum temperature of 225 ◦C. The response time was less than 1 s and the best selectivity was also obtained. The greatly enhanced gas sensing performance may be attributed to cubic/hexagonal CdS homojunction.