In this paper, we show that the sentence pattern ”X + RU (RUO) + Y + Adj” 'X + as + Y + Adj' with the meaning of a certain equal degree between X and Y, which newly developed in Middle Chinese and distributed mainly in the southern part of the country, gradually replaced the earlier sentence pattern ”X + Adj + RU (RUO) +Y” 'X + Adj + as + Y', which prevailed in Old Chinese and denotes a similar meaning to the former sentence pattern. In this paper, we call the old sentence pattern ”type B” and the new sentence pattern ”type A”. Although there are examples in Middle Chinese, which are in the form of type A in Old Chinese, they are in fact different from type A in structure because of the way of reanalysis. As for the mechanisms by which type B replaced type A, we suggest there are at least two factors that might be responsible for it: the disappearance of the markers of nominalization and the prosperity of compounds of antonyms in Middle Chinese.