Current educational reform in K-12 schools in this nation is much driven by the No Child Left Behind Act. One central goal of NCLB is to "bring all students (including special education children) up to grade level in reading and math, to close the achievement gap and to hold schools accountable for results"(ed.gov). Varied innovative efforts are being undertaken by school systems to ensure they meet the NCLB mandate, and one such instructional strategy is co-teaching (McDuffie, Mastropieri, & Scruggs, 2009). Co-teaching may have different names such as team teaching, parallel teaching, alternative teaching, or collaborative teaching, but the essential is having two qualified teachers in the classroom working together to plan, teach, and assess students learning (Dieker & Murawski, 2003; Bouck 2007). But research on effects of co-teaching in elementary schools is emerging and inconclusive (Dugan & Letterman, 2008). The purpose of this action research project was to compare the effects of co-teaching vs. solo teaching on students' academic achievement in the elementary school, specifically this research focused the comparative effects on fifth graders' math achievement. Participants of this study were the students of two fifth grade math classes in one elementary school, one with a regular education teacher (solo-teaching) and the other with the same regular education teacher and a special education teacher (co-teaching). Participants' math achievement as measured by both standardized and non-standardized tests including Unit tests, 120 Day test, CRCT, and ITBS) was analyzed for comparison. A series of unpaired t-tests was conducted comparing the various math scores of students in the two classes, solo teaching and co-teaching. No statistical significant difference was found, but noticeable differences in 9 out the 10 sets of math scores did seem to favor co-teaching. Results of the study indicate that co-teaching appears to have more positive effect on students' achievement in elementary schools. Due to limitations of this study (intact classes, no random selection of subjects, small sample size, teacher as researcher), further research is needed. (Contains 5 tables.)