We report in situ measurements of magnetoelastic coupling, B 2 , and stress, , in Co 35 Pd 65 alloy films epitaxially grown on a Cu/Si͑001͒ 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 B 2 increases from 0.72ϫ10 7 J/m 3 at 2 ML to 3.31ϫ10 7 J/m 3 at 10 ML. A second-order strain correction of B 2 ϭB b ϩC 1 ⑀ϩC 2 ⑀ 2 rather than a first-order one of B 2 ϭB b ϩC 1 ⑀ provides a better fit for the observed behavior of B 2 versus film strain, ⑀, where B b is the bulk value. The relationship between B 2 and ⑀ observed in the present study reveals that the second-order correction is crucial for understanding the dependence of B 2 on ⑀ in an ultrathin regime.