The cross between a red-mutant (R, ♀) and a wild type (W, ♂) of Pyropia haitanensis in laboratory demonstrated that its gametophytic blades were mainly monoecious. However, sexual observations showed a contrary result in the wild populations of this species. In order to clarify the real sex of the wild blades, some of the key issues related to this discrepancy were studied in this paper. Sequencing analysis of 5.8S rDNA region demonstrated that the monoecious and dioecious blades in wild populations of P. haitanensis were the same species. In nature, the heterozygous conchocelis produced in the cross between a red mutant and the wild type was cultivated, and the conchospores released from mature conchocelis were cultivated into F1 blades. Microscopic observation showed that two parental color sectors (W sector and R sector) and two recombinant color sectors (R′ sector and W′ sector) appeared in F1 blades, and 95.6% of the young blades (about 5 mm in length) were linearly sectored with 2-4 color-sectors. Besides, when those young blades grow up, each mature color-sector in the sectored blade was unisexual (either male or female). Therefore, the color-sectored blades were also the sex-sectored blades. Those findings tallylied with those obtained in laboratory, indicating that neither the position of meiosis nor segregation mode of the parental sex was change in natural conditions. However, during subsequent development of the young blades, one or two basal color-sector(s) was arrested severely. As a result, they formed very small areas on the blade which were hardly discerned by naked eyes. The area composed the rhizoid or basal part of the adult blade which could hardly maturate. Nearly entire blade was derived from one or two top color-sector(s), therefore color-sectored blades dropped to 42.1%. However, the uppermost sector took very large area of the blade and its maturity was much earlier than the middle and basal part. As a result, in the beginning and middle stages of the maturity, almost all the mature parts of the blades were in the upmost sector, and their phenotype showed unisexual even though they were monoecious actually. At the end of maturity of the blades, the middle sectors started to maturate, so the percentage of monoecious blades increased significantly. The results above showed that the wild blades of P. haitanensis in nature were also mainly monoecious. However, the probability for observation of its real sex was greatly reduced because of the uneven development of the tetrad and the asynchronous maturity of the top, middle and base of the blades. Due to these factors, one-time sampled by naked eyes often revealed that the wild blades were dioecious phenotype, with a few exceptions of monoecious phenotype.