We report in situ measurements of magnetoelastic coupling, B2, and stress, σ, in Co35Pd65 alloy films epitaxially grown on a Cu/Si(001) substrate in a thickness range of 1–10 ML by means of a highly sensitive optical deflection-detecting system. It was found that the value of B2 increases from 0.72×107 J/m3 at 2 ML to 3.31×107 J/m3 at 10 ML. A second-order strain correction of B2=Bb+C1Є+C2Є2 rather than a first-order one of B2=Bb+C1Є provides a better fit for the observed behavior of B2 versus film strain, Є, where Bb is the bulk value. The relationship between B2 and Є observed in the present study reveals that the second-order correction is crucial for understanding the dependence of B2 on Є in an ultrathin regime. © 2001 American Institute of Physics.