Purpose::Obesity is a complex multifactorial disease with increasing prevalence worldwide. The present study was conducted, since there were different results on the effect of obesity on the prognosis of patients with moderate and severe brain trauma, and the issue was less investigated.Methods::The present descriptive-analytical study was conducted in 2 hospitals, Al-Zahra and Kashani in year 2022. Patients with Glasgow coma scale (GCS) score of 9 -12 (moderate concussion) and patients with a GCS score of < 8 (severe concussion) who consented to participate in the study were included in the study. Patients who died; had serious injuries related to the chest, abdomen, pelvis, spine, and organs, in addition to the concussion; had a part of their body amputated during the same incident; received medications; or had diseases which caused obesity like diabetes were excluded from the study. Patients’ height and weight were extracted for calculating the body mass index (BMI). Their functional independences were measured at admission and discharge according to the Glasgow outcome scaleextended (GOSE) scale. All the data were analyzed in SPSS 26.Results::This study examined a total of 287 traumatic brain injury (TBI) patients (251 with moderate concussion and 36 with severe concussion). In total, 91 (36.3%) patients with moderate TBI had a lower BMI, and 14 (38.9%) patients with severe TBI had a constant BMI. There was a significant difference between the mean changes of BMI and the GOSE, functional independence measure (FIM) motor ( p =0.006), FIM cognitive ( p =0.023), and FIM total scores ( p =0.002) in patients with severe TBI; however, significant difference was found only between the mean changes of BMI, GOSE and FIM motor scores ( p=0.001) in patients with moderate TBI. Conclusion::BMI is a risk factor affecting treatment results in patients with TBI, which should be controlled.