As a preliminary study for the standardization of K-Bayley-4., this study looked at the reliability and vatidity of instruments tailored to Korean culture and language. A model was created and the item difficulty, reliability, and validity of five subtests were verified. After administering K-Bayley-4 preliminary form to 257 infants and toddlers aged 16 days to 42 months and 30 days divided into 17 age groups, results were revea;ed as follows. First, difficulty analyses showed that cognitive, receptive communication, expressive communication, fine motor, and gross motor subtests all showed a tendency of difficulty to increase depending on the order of the items, similar to the order of the original items. In addition, the starting point and baseline items for most of 17 age groups were appropriate, exceeding a 90% pass rate, but some starting and baseline items needed to be rearrangaed. Second, the Spearman-Brown reliability coefficients for each subtest and inter-rater agreement were high, similar to the US standardization study. Third, internal structure analyses revealed the validity of the K-Bayley-4,. The correlation between subtests within the same scale (e.g., between receptive communication and expressive communication subtests or between fine motor and gross motor subtests) was very high, but correlations between subtests in different scales(e.g., receptive communication and fine motor subtest) were moderate. Therefore, it showed a tendency to have high intercorrelations with theoretically related subtests(convergent validity) and low intercorrelations with tests that were theoretically unrelated to each other (discriminant validity), showing an internal structure consistent with the that of Bayley-4.. This study showed that five subtests of K-Bayley-4 are tools that can reliably and validly measure the cognitive, language, and motor development of Korean infants and toddlers.