A mathematical model which displays the relation between the vibro-acoustic modulation index and the crack contact area was developed. Based on the analysis of crack faces response to the resultant of applied load and plasticity induced crack closure stress, a procedure of on-line approximating the opening and closure stress was proposed. To study the performance of this measuring method experimentally, the on-line vibro-acoustic modulation tests were carried out on a central cracked aluminum alloy fatigue sample. Analysis of the experimental results suggests that the vibro-acoustic modulation technology is not only capable of measuring the fatigue crack opening and closure stresses, but also capable of analyzing the crack closure behavior, even for physically short cracks.