Orbital free entropy χorb should be regarded as a candidate of microstate definition of Voiculescu’s free mutual information i ∗ . In fact, it measures the “degree of free independence” among hyperfinite subalgebras inside a fixed tracial W∗ -probability space and satisfies the natural formula χ(X1, . . . , Xn) = χorb(X1, . . . , Xn) +Xn k=1 χ(Xk). In the talk, I’ll discuss it and its dimension counterpart δ0,orb (like free entropy dimension δ0) together with some recent progress. The materials are mainly due to a joint work with F. Hiai and T. Miyamoto.