For the syntheses of LiNi1−yFeyO2 (0.000 ≤ y ≤ 0.300), mixtures of the starting materials with the desired compositions were preheated in an air atmosphere at 400 °C for 30 min and calcined in air at 700 °C for 48 h. The phases appearing in the intermediate reaction steps for the formation of lithium nickel oxide are deduced from the DTA analysis. XRD analysis, FE-SEM observation, FTIR analysis and electrochemical measurement were performed for the synthesized Li1−z(Ni1−yFey)1+zO2 (0.000 ≤ y ≤ 0.300) samples. The samples of Li1−z(Ni1−yFey)1+zO2 with y = 0.025 and 0.050 have higher first discharge capacities than Li1−z(Ni1−yFey)1+zO2 with y = 0.000 and better or similar cycling performance at the 0.1 C rate in the voltage range of 2.7–4.2 V. Similar results have not previously been reported except for Co-substituted LiNiO2. The sample Li1−z(Ni0.975Fe0.025)1+zO2 has the highest first discharge capacity (176.5 mAh g−1). Rietveld refinement of the XRD patterns of LiNi1−yFeyO2 (0.000 < y ≤ 0.100) from a starting structure model [Li,Ni]3b[Li,Ni,Fe]3a[O2]6c showed that cation disordering occurred in the samples.