With designed genespecific primers on basis of Clone 18 sequence screened out of a suppressive subtracted cDNA library from the male gametophyte of Laminaria japonica, a full length cDNA (GenBank accession No: EF490312) was cloned by use of rapid amplification of cDNA ends (RACE). It was composed of 2087 bp in length including 118 bp 5′-untranslated region (UTR), 694 bp 3′-UTR with poly A at this end and 1 275 bp open reading frame (ORF). Its homology with Guillardia theta nucleomorphencoding CbbX protein (GenBank accession number: CAB65663) reached 66% in peptide sequence. The deduced protein of L. japonica cbbX gene contained 424 amino acids with the first 19 ones from N-terminal on constituting a signal peptide. The matured protein was composed of 405 amino acids after endonuclease restriction digestion with a putative molecular weight of 45.26 ku and pI at 5.28. The coding region of cbbX gene was interrupted by eight introns with all splicing sites well matching a GT-AG rule. There was no difference between female and male gametophyte cbbX genes in their cDNA or DNA sequences. From 184 Gly of the encoded protein, there was a Walker ATPbinding motif. The deduced CbbX from L. japonica gametophytes was clustered with cryptophyte nucleomorphand rhodophyte nucleusencoding CbbX which was significantly different from the chloroplastencoding one according to the constructed neighborjoining phylogenetic tree. It was supposed, therefore, that the cloned cbbX gene from L. japonica gametophytes was possibly encoded by nucleus genome. The transcription level of cbbX gene in male gametophytes was proven significantly higher than that in females by quantitative real-time PCR, and the diurnal transcription patterns were a little different between males and females, both of which confirmed that cbbX gene was a differentially expressed one in L. japonica gametophytes. This research lays a foundation for the function identification and subcellular location of cbbX gene.