Copy number variations (CNVs) are an important source of genetic variation, which can affect a wide range of economic traits by diverse mechanisms. KMT2D (Lysine methyltransferase 2D) is an important positional candidate for growth traits. Quantitative trait loci (QTLs) with large effects on economically important traits cover the KMT2D gene. The KMT2D gene overlays a CNV within its exons, hence it was chosen as a crucial candidate gene to study the association between CNV and growth traits. Further, KMT2D, a major mammalian histone H3K4 mono-methyltransferase, plays a critical role in regulating development, differentiation, metabolism, and tumor suppression. Therefore, we proposed the hypothesis that KMT2D CNV may have phenotypic effects on sheep growth traits. In our study, KMT2D CNVs in three Chinese sheep breeds were detected by quantitative polymerase chain reaction (qPCR), and the loss copy was found to be the dominant genotype. Association analysis between growth traits and KMT2D CNV was also performed, which revealed that individuals with the median copy showed better performance than those with the loss copy in all three breeds. This research suggested that KMT2D CNV can be used as a promising marker for sheep molecular breeding.