Background: Childhood obesity prevention studies have yielded disappointing results. Understanding intervention fidelity is necessary in explaining why interventions are (or are not) successful and ultimately improving future intervention. In spite of this, intervention fidelity it is not consistently reported in the obesity prevention literature. The purpose of the current study was to develop and utilize a coding protocol to objectively assess intervention fidelity in a phone-based obesity prevention study for parents of preschool-aged children. Findings: Both interventionists and independent coders completed session fidelity measures including time spent on target areas (media use, physical activity, etc.) and components of goal setting quality. Coders also rated participant engagement. Agreement between ratings by interventionists and coders, fidelity levels and changes in fidelity components over time is presented. Coders and interventionists showed high agreement when reporting time spent discussing different target areas. Interventionists consistently rated themselves higher than independent coders on measures of goal quality. Coder ratings of session quality were initially high, but some components declined slightly across the eight sessions. Conclusion: Future directions for intervention fidelity measurement and analysis are discussed, including utilizing changes in fidelity measures over time to predict study outcomes. Obtaining a more in-depth understanding of intervention fidelity has the potential to strengthen obesity interventions.