Objectives: In this study, we aimed to investigate whether there was a relationship between deformity at the fracture line/eccentric placement of the nail, and union time (UT) in isthmal diaphyseal femoral fractures treated with intramedullary nails. Patients and methods: Between September 2017 and December 2020, a total of 61 patients (38 males, 23 females; median age: 47 years; range, 23 to 62 years) with closed femoral shaft fractures who underwent antegrade nailing were retrospectively analyzed. The following parameters were examined: (i) amount of angulation in the fracture line (AFL), (ii) varus/valgus of the fracture line (VAFL), (iii) amount of deviation of the distal tip of the nail from the femoral notch (DDT), (iv) medial lateral orientation of the distal nail relative to the notch (MLON), (v) number of fracture parts (NFP), and (vi) UT. Results: The causes of injury were high-velocity traffic accidents in 42 patients and falls in 19 patients. The median surgical delay was 4.5 (range, 2 to 8 days). The median follow-up time was 37 (range, 12 to 57) months. There was a moderate, statistically significant and positive correlation between UT and AFL, DDT, and NFP (r=0.486, p<0.001). The difference in UT according to MLON (p=0.002) was statistically significant. Conclusion: Our study results suggest that impaired weight-bearing force and translational force may cause impaired healing. Thus, angulation of the fracture line and eccentric nail placement may delay fracture union. We recommend using bold screws to ensure that there is no deformity in the fracture line and to fully center the nail inside the distal bone.