Purpose: The goal of this study was to examine online and off-line sentence processing using Korean language relative clause sentences between children with specific language impairment (SLI) and children with typical development (TD). Method: Twenty-four children with TD and 19 children with SLI participated in this study. Children completed online and off-line sentence-processing tasks using relative clause sentences. The response time (RT) data obtained from the online processing task were analyzed at each word position and between adjacent words for items answered both correctly and incorrectly on the off-line comprehension task. A linear mixed-effects model and a generalized linear mixed effects model were used to analyze the performances on the online/off-line sentence-processing task between the two groups. Results: The results revealed that the processing pattern of RTs on the online processing task differed between the two groups, such that the SLI group did not show the predicted RT increase while the TD group did. Also, the SLI group processed each word with comparable or faster reading rates than the TD group. On the off-line comprehension task, the SLI group performed poorly compared to the TD group. Conclusions: Processing of syntactically complex sentences differed between the TD and SLI groups, such that the SLI group had lower accuracy on the off-line comprehension task and was less efficient on the online processing task as compared to the TD group. These results mainly support the syntactic deficit account in children with SLI.