This paper presents the findings from a randomized control trial study of reading/literacy-integrated science inquiry intervention after 1 year of implementation and the treatment effect on 5th-grade low-socio-economic African-American and Hispanic students' achievement in science and English reading. A total of 94 treatment students and 194 comparison students from four randomized intermediate schools participated in the current project. The intervention consisted of ongoing professional development and specific instructional science lessons with inquiry-based learning, direct and explicit vocabulary instruction, and integration of reading and writing. Results suggested that (a) there was a significantly positive treatment effect as reflected in students' higher performance in district-wide curriculum-based tests of science and reading and standardized tests of science, reading, and English reading fluency; (b) males and females did not differ significantly from participating in science inquiry instruction; (c) African-American students had lower chance of sufficiently mastering the science concepts and achieving above the state standards when compared with Hispanic students across gender and condition, and (d) below-poverty African-American females are the most vulnerable group in science learning. Our study confirmed that even a modest amount of literacy integration in inquiry-based science instruction can promote students' science and reading achievement. Therefore, we call for more experimental research that focus on the quality of literacy-integrated science instruction from which middle grade students, particularly low-socio-economic status students, can benefit.