Apple had after international media reports on work practices at the supplier Foxconn, a unit of Taiwan's Hon Hai Precision Industry.
In a statement, Apple said its code been violated in some cases, but insisted the overtime was voluntary.
"We found no instance of forced overtime," Apple said. "We, however, found that employees worked longer hours than permitted by our code of conduct."
Foxconn had now acted to enforce work weeks of 60 hours and measures had been put in place to track compliance, the statement said.
Hon Hai spokesman Edmund Ding told Reuters in Taipei that the Apple statement gave "a very detailed explanation of the situation" and was "fair." He added that the company "has not violated any labor standards."
Cases of unreasonably long working hours, child labor and even abuse have been uncovered in some Chinese factories run by local or overseas manufacturers, but few have involved high-profile firms.