Objective: The aim of the study was to find the effect of ankle foot orthosis (AFO) on energy expenditure (EE) during walking in children with spastic diplegia (SD) due to cerebral palsy (CP). Materials and methods: Twenty-six subjects with SD CP who could ambulate with and without the help of orthosis were selected after screening with inclusion criteria. The EE of these patients during ambulation with and without orthoses was measured using the K4b² machine. Statistical analysis was performed with the help of Epi Info^TM 3.5.3. t-test, chi-square test, and correlation tests were used to study the parameters of oxygen cost, oxygen pulse, heart rate, and EE. Results: The use of AFO resulted in decreased EE in 73% of patients in the first day itself. In the sequential assessment, these patients showed further decrease in EE with AFO use. The patients who initially showed an increase of EE also showed a decrease in the rise in EE during walking in their subsequent analysis. Oxygen cost also showed a positive correlation with EE. Heart rate was not much related to use of AFO; rather, it showed an increase in the assessment done second in the same day (among walking with and without the AFO). There was no linear relation or correlation between O₂ pulse and other parameters of the study. Conclusion: The AFOs are an important intervention in the treatment of SDs. It can help decrease the EE and improve the speed of walking in most patients when prescribed along with other needful therapies like exercises, antispastic medications, etc. O₂ cost showed positive correlation with EE analysis. In patients with increased EE with AFO use, the mean difference between EE with and without AFO decreases, suggesting long-term benefits that AFO could offer. A study of longer duration with more patients should be conducted to view the full benefits of AFO in children with SD CP.