This study investigates how EFL learner error patterns and frequencies differ in writing depending on students' language proficiency levels. A total of 152 Korean EFL students' essays were examined. The writing samples were first scored holistically on an impressionistic basis; then, based on the scores, the samples were grouped into low- and high-proficiency groups. The study identified 13 errors based on prevalence and significance. The study calculated the frequency of each error type committed by each group and compared the results using t-tests. A survey of English teachers was also conducted (N=32) to rate the gravity of each error and to compare the error gravity produced by each group. The result showed that low-proficiency group produced errors much more frequently, and the errors were more serious ones with high error gravity. The errors committed by the high-proficiency group were usually complex ones and occurred in more complicated contexts. The study also found that the errors with less gravity occurred more frequently in both groups and that they tended to be more persistent. The study indicated that although the students were aware of significance of errors, correcting the errors was beyond their current English capability.