LincRNA-p21 is a long intergenic non-coding RNA (LincRNA) gene reported to activate the transcription of the adjacent Cdkn1a (p21) gene in cis. The importance of the enhancer elements in the LincRNA-p21 gene region has also been reported; however, the involvement of the LincRNA-p21 transcripts in regulating Cdkn1a in vivo is still unclear. In this study, we used a LincRNA-p21-trapped mouse line (LincRNA-p21Gt) in which βgeo was inserted into intron 1, and all enhancer elements were retained. In LincRNA-p21Gt/Gt mice, the transcription of LincRNA-p21 was repressed due to the βgeo sequence, and the expression of exon 1 of LincRNA-p21 was restored through its deletion or replacement by another sequence, and Cdkn1a expression was also upregulated. Furthermore, regardless of the full-length transcripts, the expression of Cdkn1a correlated with the transcription of the exon 1 of LincRNA-p21. This result indicates that the LincRNA-p21 transcripts are not functional, but the transcriptional activity around exon 1 is important for Cdkn1a expression.