Lithium diffusion is a key factor in determining the charge/discharge rate of Li-ion batteries. Herein, we study the tracer diffusion coefficient (D*) of lithium ions in thec-axis oriented LiCoO2thin film using secondary ion mass spectrometry (SIMS). We applied a step-isotope-exchange method to determineD* in the Li-extracted LixCoO2. The observed values ofD* ranged from 2 × 10−12to 3 × 10−17cm2s−1depending on the compositions in the range of 0.4 <x< 1.0. Approaching the stoichiometric composition (x= 1.0),D* decreases steeply to the minimum, which can be explained by the vacancy diffusion mechanism. Electrochemically determined diffusion coefficients corrected by thermodynamic factors are found to be in good agreement withD* determined by our method, over a wide range of compositions. Thec-axis diffusion was explained by the migration of Li+ions from one layer to another through additional diffusion channels, such as antiphase boundaries and a pair of Li antisite and oxygen vacancies in cobalt oxide layers.