Since Convention on the Rights of the Child of the United Nations are released, children's best interests becomes the highest principle for every state having legal system to deal with children's affairs. So we can say that the first serious mission of the subject of modern parent and tot's relation is protecting minor children's interests, then keeping the marriage and family peace and stable identity relations and etc. is the next. And in order to reach the above-mentioned purpose, the numerous countries have begun to examine their law of the marriage and family, even some advanced countries have abolished the differentiation between legitimate and illegitimate children too, in order to reach the demand for this two equalization. Now people of our country and mainland China ( what follows we call them two sides) contact with each other frequently, and then the extramarital sexual intercourse of people of two sides becomes numerous, and the quantity of the legitimate children also increases year by year. Base this, the announcement of the Equal Principle between legitimate and illegitimate children in mainland China marriage law has a deep meaning, but when we review our country, we find that our law about children's affairs has no response for the above-mentioned things. Even now our law about children's affairs still maintain the differentiation of legitimate and illegitimate children and also especially discriminate against the children of the mainland China in "The ordinance of people's relationship between Taiwan and mainland China ". So this thesis's aim is checking the legal system related to legitimate and illegitimate children of two sides by the standard namely with the children's best interests and look over them to find they observe the standard namely with the children's best interests or not. By the way, this thesis will try to find the reason and the cause that helping legal system about legitimate and illegitimate children stand out ,and try to find the real way to make legitimate children equal with illegitimate children in the modern status which has varied family type and varied pattern of both sexes getting along with each other. To this end, the thesis want to set up a new ¡§legitimate children presumption¡¨system to adapt to the above-mentioned purpose, and doesn't make children unequal just because their parents aren't husband and wife , and our law ¡]or mainland China's law¡^about children's affairs will juxtapose advanced countries.