Type I error rates of the likelihood ratio test for the detection of differential item functioning (DIF) in the partial credit model were investigated using simulated data. The partial credit model with four ordered performance levels was used to generate data sets of a 30-item test for samples of 300 and 1,000 simulated examinees. Three different combinations of sample sizes of reference and focal group comparisons were simulated under two different ability matching conditions. One hundred replications of DIF detection comparisons were simulated for each of six conditions. Type I error rates of the likelihood ratio for all six conditions were with theoretically expected values at each of the nominal alpha levels considered. (Contains 25 references.) (Author/SLD)