High selfing rates are relatively common in island plant species compared to species of mainland habitats. In such small island populations, the effect of inbreeding depression (ID) may be reduced by the purging of deleterious recessive alleles. Decreasing the effect of ID might cause the reproductive shift in self fertilization in island plants. We investigated the selfing rate and ID of island and mainland populations of two closely related Clerodendrum species, the partial self-incompatible widespread species C. trichotomum and the self-compatible insular species C. izuinsulare. Half of all seeds collected from both mainland and island population were incubated for germination. To quantify ID, heterozygosities of the germinated seedlings were compared with that of remaining seeds. Selfing rates of C. izuinsulare were approximately four times higher than of C. trichotomum. Heterozygosity of C. trichotomum was not different between seeds and seedlings. For C. izuinsulare, heterozygosity was lower for seedlings, suggesting outbreeding depression. Insular self-compatible C. izuinsulare may have co-adapted gene complex to inbreeding, and that adaptive ability may be disturbed by heterozygous loci.