The objectives of this research were 1) to study the relationship between student factors, English language instructor factors, and environment factors with students' achievement in English language learning and 2) to develop equations to forecast factors affecting students' achievement in English language learning. The population used in this research was 415 students of Thailand National Sports University of Central Region enrolled in the English for Communication Course in the second semester of the academic year 2019. The sample size was determined using the Krejcie & Morgan table. A sample of 200 was obtained and a simple random sampling method was used based on the proportion of the population on each of the five campuses. The research instrument was a five-rating scale questionnaire comprising items on student factors, English language instructor factors and environment factors, with a reliability of the whole questionnaire of 0.973. The statistics used in the data analysis were fundamental statistics, Pearson correlation analysis, and stepwise multiple regression analysis. The major findings indicated that there were four factors affecting students' achievement in English language learning with statistical significance at the 0.01 level namely, attitude variables towards learning English language course (X[subscript 1]), instruction management variable (X[subscript 4]), student availability variable (X[subscript 2]), and classroom environment variable (X[subscript 7]). The four factors could jointly explain the variance of students' achievement in English language learning by 52.50% and could be used to forecast students' achievement in English language learning in the form of raw scores and standard scores as follows: Y' = 1.125 + 0.320 X[subscript 1] + 0.139 X[subscript 4] +0.124 X[subscript 2] + 0.096X[subscript 7] Z' = 0.375 [subscript z1] 0.165 [subscript z4] + 0.145 [subscript z2] + 0.128 [subscript z7]