Objective: This study sought to obtain basic data concerning the Korean standardization of the Bayley-4 assessment tool; this was achieved by reviewing the content validity and reliability of the language scale designed in accordance with the development of Korean infants and Korean culture. Methods: First, a translation/reverse translation process was performed on the Bayley-4 language scale (receptive communication, expressive communication). Afterwards, the language scale of the K-Bayley-4 was developed by revising, supplementing, and deleting the questions through a process of expert review and an initial preliminary survey (n = 68). After conducting a second preliminary survey of 257 infants and toddlers aged 16-43 months in Seoul, Gyeonggi-do, Chungcheong-do, Gyeongsang-do, and Jeolla-do, the content validity, question difficulty, passage rate, and reliability of the language scale were analyzed. Results: As a result of the second preliminary study, the language scale of the K-Bayley-4 consists of 44 receptive communication and 41 expressive communication items. These results produce a similar order to the existing questions, with the exception of certain items which diverge due to differences in language development and Korean grammar. The starting point of the age group was adjusted from 3 to 6 points for the expressive communication category only. The reliability of the language scale was found to be within an appropriate range. Conclusions: It can be seen that the language scale of K-Baley-4 can appropriately evaluate the language development of Korean infants and toddlers; that is, the receptive and expressive communication.