It has been reported that children's comprehension of complex structures is affected by temporary ambiguity. Yet, much less is known about its effect on children's comprehension of relative clauses (RCs). To fill this gap, the present study focused on Mandarin RCs, where temporary ambiguity often plays a role in the comprehension of object-extracted RCs. More specifically, we directly manipulated the (non)existence of temporary ambiguity in object-extracted RCs and compared 3- to 5-year-olds' understanding of object-extracted and subject-extracted RCs. Using the Truth Value Judgment Task, we found a clear developmental trajectory of preschoolers' abilities to comprehend the RCs. The 3-year-olds could correctly understand the object-extracted RCs only when there was no temporary ambiguity in the sentence, while the 4- and 5-year-olds exhibited successful comprehension regardless of the (non)existence of temporary ambiguity. In addition, when temporary ambiguity was not present, object-extracted RCs were generally easier than subject-extracted RCs for preschoolers to comprehend. Taken together, the findings inform us about the role of temporary ambiguity in RC comprehension, and point to the necessity of taking into account this factor when examining children's sentence comprehension performance.