To compare energy expenditure of ambulation on even indoor and uneven outdoor surface in CP children as well as between CP and asymptomatic children. Objective: To find out and compare physiological cost index on even indoor and uneven outdoor surface during walking in spastic CP and asymptomatic children. To correlate gross motor function and PCI. Method: 30 CP children and 30 asymptomatic children between 3 to 17 years walked for 6 minute on indoor even and uneven outdoor surfaces. Their energy expenditure was measured by physiological cost index. Results: Independent ‘t’ test and paired ‘t’ test were used for analysis. There was statistically significant difference between normal and CP children also between indoor even and outdoor uneven surfaces. Indoor performance better than outdoor performance under both conditions. Conclusion: children with CP having more energy expenditure than normal children on both surfaces.