A/B testing is a widely-used experiment strategy, in which a developer implements multiple variations, runs them in parallel, and analyzes their effects. It is needed to assign each variation to users or sessions in A/B testing. By using AOP (Aspect Oriented Programming), the assigning operation and the features of a software can be implemented separately. Here we propose the solution of automated A/B testing. Variations are expressed declaratively in <I>Archface-U</I>, an interface which expresses candidates for implementation. In the expression, assignment ratio is described with the annotations, which realize a declarative description of the entire A/B testing strategy. Based on the description, we generate an aspect which automate the assigning operation in A/B testing.