This study examined the notational characteristics of case particles in the 17th century royal Korean old vernacular letter, ‘Sukmyeong·Sukhwi Sinhancheop’. It examined the appearance of the honorific subjective case, ‘∼으로겨오셔/으로겨□ 오셔’, that is not found in other Korean old vernacular letters, the apperance of subjective case, ‘가’ in the Korean old vernacular letter written in the middle and late 17th century, ‘ㆍ’ appeared as ‘□’ and ‘□’ when the noun final is the open syllable despite the change of ‘ㆍ>ㅡ’ and the weakening of vowel harmony in the 16th century, and the locative case being same as the genitive case and prevailance use of ‘의’ being as an unusual locative case in the 15th century. Through this, it discussed that as the ‘가’ appeared in the middle and late 17th century, the subjective case ‘ㅣ’ in the 15th century weakened in the process of unifying the subjective case. For subjective case and the objective case, when the noun final is a syllable, ‘□’ and ‘□’ were used mainly at some point in the unification process for the objective case ‘□/ 를>를’ and subjective case ‘□/ 는>는’. For the locative case, in the process of unifying to ‘에’, ‘의’ which is same as the genitive case, was used more often than the ‘에’ at some time.(Chungnam National University)