‘難nan, 易yi/容易rongyi’ are a pair of antonyms adjectives that are frequently used in modern Chinese. They have many similar uses, but they have differences in terms of meaning, lexical, part of speech and grammatical functions. First of all, the difference between the two in terms of word formation ability is significant. ‘難nan’ often forms a two-syllable word with other monosyllabic morphemes or words, gradually develops into a morphemes, and the ability to form words is relatively strong, On the other hand, word formation ability of ‘易yi’ is very weak; the two show a relatively formulaic symmetry in grammatical function, but there are partially differences. This paper mainly analyzes the ability to form words and grammatical the symmetry and asymmetry of ‘難nan’ and ‘易yi/容易rongyi’, and explains reasons to form asymmetry of them.