Tho1 is a RNA-binding protein that assembles co-transcriptionally onto the nascent mRNA and is thought to be involved in mRNP biogenesis and mature mRNA export to cytoplasm in budding yeast. In fission yeast Schizosaccharomyces pombe, a homologue of THO1 (spTho1) was identified based on sequence alignment. A deletion mutant in a diploid strain was constructed by replacing one of spTho1-coding region with an ura4+ gene using one-step gene disruption method. Tetrad analysis showed that the spTho1 was not essential for growth. The spTho1 mutant did not show any defects of bulk mRNA export. However, over-expression of spTho1 from strong nmt1 promoter caused the growth defects and accumulation of poly(A)$^+$ RNA in the nucleus. These results suggest that spTho1 is involved in mRNA export from the nucleus to cytoplasm though it is not essential.