This paper was written to study effects of spread of adoption in the late Joseon Dynasty on succession of the existing illegitimate children. And for this, ‘yehu Deugrok(Registration Document of Adopted Sons)’, the collection of adoption examples compiled by Yejo, was used as major historical records.The adoption requirements specified by the adoption system, that is, the system that adoption is possible if wives and second wives do not have any son continued in effect even after the 17th century. However, there were demands to try to make adoption succeed to families instead of the succession of illegitimate children as they got to be low in position due to legitimate and illegitimate children discrimination. Especially, the people who were power elites tended to adopt sons obtaining special permission from kings. And for primogeniture, there was the aspect that adoption was restrictively done. Therefore, ways to adopt sons were opened if families wanted even though there were illegitimate sons.There were often the cases that legitimate sons deprived illegitimate sons of their rights of succession. It's because legitimate sons tried to deprive illegitimate sons of their rights of succession by using disparity in social status despite that the illegitimate sons already performed services after they become successors. Accordingly, illegitimate sons tried to keep their rights of succession through disputes and furthermore, they requested the nation to Successor Certificate to get the credit for the rights of succession.Like this, there were serious conflict situations if the process to select successors cannot reach an understanding as the spread of adoption meshed with illegitimate sons' protection of rights of succession. For the conflict aspects, first, illegitimate sons reacted against adoption. Second, adoption was seriously considered but the succession of illegitimate sons was selected. So candidates of adoption reacted against the succession. Third, legitimate sons tried to deprive illegitimate sons of their rights of succession even though their succession was done.Eventually, the spread of adoption in the late Joseon Dynasty widened options when families with only illegitimate sons tried to decide succession ways. Therefore, succession of illegitimate sons or adoption could be done according to choices of families. However, there were the aspects which involved conflicts which option was chosen. So it is thought that agreement in families was positively necessary.