The present study analyzed the process of arriving to the use of ‘要是’ as a subjunctive mood conjunction. The use of ‘要是’ was divided into two aspects, ‘要’ and ‘是’, during the analysis. Firstly, the study examined the semantical characteristics of ‘要’ that enabled the expansion of function from an auxiliary verb to a conjunctive particle. The fundamental semantic function of ‘要’ could be summarized using the irrealis mood concept. The auxiliary verb ‘要’ indicates a future tense that is in an unreal time and unreal domain. The conjunctive particle, ‘要’ further evolves the potentiality of irrealis, expanding its meaning to the subjunctive mood. Next, the study examined the reasons for including ‘是’ in the subjunctive mood conjunction ‘要是’. This study analyzed the fixture of ‘要’ and ‘是’ being used together from the framework of lexicalization. The reason why ‘要是’ was able to be lexicalized was because ‘是’ first became grammaticalized. Diachronically, ‘是’ was developed in the order of a demonstrative-copula-focus marker. The ‘是’ in ‘要是’ is a further grammaticalization, in that it functions as a component in a word. More specifically, the conceptual basis that enabled ‘是’ to be included in a subjunctive expression is because ‘是’ has a temporal persistence that connects the past, the present, and the future. Furthermore, although ‘是’ accompanies a noun component as an object, functioning as a copula verb, this changed as the scope of the subsequent component expanded into the verb component. Subsequently, ‘是’ lost its lexical meaning and phonetic stress and became dependent on ‘要’ to create a subjunctive mood conjunction.