This study aims to describe the acquisition of the first language of children aged three years based on phonological aspects including, the acquisition of vowels, acquisition of consonants, and factors that influence the acquisition of phonology in children. This research uses a descriptive qualitative approach. The research data is in the form of children's speech. The data source was obtained from a three-year-old child named AMS who grew up in an environment with Madurese first language. At the age of three years, AMS is going through a golden period or a critical period. During this golden period, children begin to be able to carry out the language acquisition process. Data collection was carried out using observation and interview methods. After that, the data obtained were then analyzed through three stages, namely the data reduction stage, the data preparation stage, and the conclusion drawing stage. The findings of the study indicate that at the age of three, children are already fluent in the Madurese language, but there is little influence of Indonesian on the child's Madurese language. At this age, children get a phonemic inventory of the Madurese language, including Indonesian. The child has mastered the vowels [a], [i], [u], [e], and [o] along with their allophones. In addition, children also master several consonant sounds, including [b], [p], [m], [n], [w], [j], [k], [g], [p], [ l], [ŋ], [ɲ], [h], [tʃ],, [ʃ] and [ʤ]. Meanwhile, the sounds [r], [s], and [f] cannot be pronounced properly even though they are located at the beginning, in the middle, or at the end of the word. Based on the results of observations, the limitations of children in pronouncing some of these consonants are caused by the child's articulator factor which has not grown perfectly. Even though the researchers provoked consonants that had not yet appeared in the child's pronunciation, the child was still not able to pronounce the sound of the consonant phoneme. In this regard, it can be seen that the phonological acquisition of a child's first language is not determined by the year, but is strongly influenced by neurobiological factors. Therefore, it is not impossible if the acquisition of the sounds of children of the same age in the same year can be different in number, because what is universal is not the year, but the appearance of these sounds. Another factor that affects the acquisition of AMS phonology is the stimulus provided by the family and the environment around the child. Based on some of the data found, certain sounds are the result of imitation. Therefore, the acquisition of a child's first language is also greatly influenced by the environment around the child.