We previously created two PER2:: LUCIFERASE (PER2:: LUC) circadian reporter knockin mice that differ only in the Per2 3'-UTR region: Per2:: Luc, which retains the endogenous Per2 3'-UTR and Per2:: LucSV, where the endogenous Per2 3'-UTR was replaced by an SV40 late poly(A) signal. To delineate the in vivo functions of Per2 3'-UTR, we analyzed circadian rhythms of Per2:: LucSV mice. Interestingly, Per2:: LucSV mice displayed more than threefold stronger amplitude in bioluminescence rhythms than Per2:: Luc mice, and also exhibited lengthened freerunning periods (similar to 24.0 h), greater phase delays following light pulse, and enhanced temperature compensation relative to Per2:: Luc. Analysis of the Per2 3'-UTR sequence revealed that miR-24, and to a lesser degree miR-30, suppressed PER2 protein translation, and the reversal of this inhibition in Per2:: LucSV augmented PER2:: LUC protein level and oscillatory amplitude. Interestingly, Bmal1mRNA and protein oscillatory amplitude as well as CRY1 protein oscillation were increased in Per2:: LucSV mice, suggesting rhythmic overexpression of PER2 enhances expression of Per2 and other core clock genes. Together, these studies provide important mechanistic insights into the regulatory roles of Per2 3'-UTR, miR-24, and PER2 in Per2 expression and core clock function.