Abstract— <p>Present work successfully demonstrates controlled doping of transition metal Fe (0.5, 1, and 1.5%) into cubic and hexagonal lattice structure of CdS nanocrystals. The paper includes detailed studies on the structural, optical, and photocatalytic properties of CdS:Fe nanocrystals. The estimated average crystallite sizes of CdS:Fe in cubic and hexagonal phases are 5 and 3 nm, respectively as revealed by the XRD analyses. Smaller crystallite sizes of h-CdS:Fe nanocrystals result in larger band gap compared to its cubic counterpart. The variations in the band gaps of CdS:Fe nanocrystals in cubic and hexagonal phases with different doping percentages of Fe do not follow any trend. Photoluminescence (PL) spectra reveal that the crystal structure of h-CdS:Fe nanocrystals contain more crystal defects compared to c-CdS:Fe nanocrystals. In addition, the prepared nanocrystals are tested as photocatalyts by observing photodegradation of Methylene blue (MB) solution with the the prepared materials immersed in it. Hexagonal phase of CdS:Fe nanocrystals have shown better photocatalytic behaviour compared to the cubic phase owing to the presence of more defects in their crystal structures than their cubic counterparts. Doping with transition element Fe into the crystal structure of cubic and hexagonal CdS nanocrystals significantly affects their photocatalytic performances.</p>