Background: Immunization is one of the most cost-effective interventions to prevent the suffering that comes from avoidable sickness, disability and death. In India, though immunization coverage has improved over the past few years, the country still accounts for the largest number of children who are not immunized i.e. 7.4 million. So, this study was taken up to assess the immunization coverage among children in District Gautam-Budh Nagar of Uttar Pradesh and to decipher the causes behind the partial immunization/non- immunization. Methods: A cross-sectional study was conducted during June-August 2015 in an urban resettlement colony of District Gautam-Budh Nagar in Uttar Pradesh among 210 children aged 12-23 months selected using WHO 30X7 cluster sampling technique. Results: A total of 210 children were covered, of which 72.9% children were fully immunized, 19.0% children were partially immunized, whereas, 8.1% children did not receive even a single dose of any vaccine, thus constituting a total of 27.1% children with partial/no immunization. On bi-variate analysis, significant association was observed between immunization status among children and caste (p=0.047), mother’s education (p=0.01), father’s education (p=0.000) and socio-economic status (p=0.003). On multi-variate analysis, however, only father’s educational status was found to be significantly associated. The major reasons for partial/non-immunization among children were Ignorance (80.7%) followed by unavailability of vaccines (7.0%). Conclusions: Immunization coverage found in the present study is still way short of the target of universal coverage. Therefore, health planners and policy makers in our country should develop a comprehensive strategy to achieve universal immunization coverage.