A number of animal species are characterized by haplodiploidy, in which fertilized and unfertilized eggs develop into females and males, respectively. However, it is poorly known how haplodiploidy has evolved. In Hymenoptera, haplodiploidy is archived on unique sex determination systems. The most ancestral sex determination system in Hymenoptera, namely complementary sex determination (CSD), includes a single sex determining locus that has many multiple alleles. If an egg is heterozygous for the locus it develops into a female, and if it is hemizygous, or accidentally heterozygous, it develops into a male. In this session, I propose some results of my simulation indicated co-evolution of unfertilized egg development and CSD alleles, which can explain the establishment of haplodiploidy in Hymenoptera.