This study aims to find contributing variables to immigrants' difficulty of communication in Korean, and the learning period required to overcome the difficulty based on the empirical evidence. A comprehensive survey was conducted on 728 people, who were from three immigrant groups (Immigrant Worker, Marriage Immigrant, Immigrant Child) living in Korea. 461 participants' responses, with no missing values, were analyzed using Hayes(2013)'s model 8. Findings are as follows: First, Length of Residence plays a role in immigrants' Self-awareness of Difficulty through the direct path and indirect path. In addition, Self-awareness of Korean Language Ability affected it as a mediator, and Learning Period as a moderator on the direct or indirect path. Moreover, adult immigrants had higher Self-awareness of Difficulty than children upon their arrival. The difference in the degree of difficulty reported by adult and child immigrants was maintained through the learning period. Second, if an adult immigrant continues to learn Korean 4 hours a week from arrival, s/he will overcome the difficulty after 6.8 years, and a child immigrant, under the same condition, needs 2.9 years.