The Piwi−interacting RNA (piRNA) pathway is an RNA silencing pathway that represses the expression of genes and transposable elements (TE) in the gonads via binding of piRNAs to their complimentary RNA targets. Mammalian piRNAs are 26−31 nt in length and are 2’−O−methylated at their 3’ termini. Although the biogenesis of piRNAs remains unclear, Hen1, a plant microRNA (miRNA) 2’−O−methyltransferase, is known to play an important role in piRNA stabilization. Deletion of hen1 in Zebrafish reduced piRNA content and led to an exonuclease−mediated shortening of the piRNAs that resulted in female sterility. In order to understand the function of mouse HEN1, we have generated a mouse model containing a point mutation in the Hen1 gene that produces truncated and unstable Hen1 transcripts. Hen1 homozygous mutant males are sterile. They produce greatly reduced numbers of sperm with round heads, and the epididymis are virtually devoid of sperm. Deep sequencing and biochemical analyses revealed the loss of HEN1 function results in piRNA instability i.e. the presence of 3’-end truncated piRNAs and significantly increased piRNA uridylation which may lead to RNA decay. Furthermore, increased expression of several transposable element (TEs) subtypes was found in Hen1 mutants using real-time PCR, Northern blot and in situ hybridisation. Transcriptome analysis also suggested a shift of spermiogenic gene expression into meiotic cells ultimately causing male sterility. In summary, our data suggests an important role for HEN1 in regulating pachytene piRNA stability, post-natal TEs repression and the translational regulation of haploid germ cell mRNAs.