Relations of reading and math skills are well-documented. Using open randomized reading intervention data, we examined a reading intervention’s impact on math fluency and problem- solving as well as the mechanisms of reading intervention’s relation to math fluency. Twenty- eight teachers and 511 first-grade students (82% White, 6% Hispanic, 53% Female, Age M[SD] = 6.7[0.45]) were randomized to reading treatment or a math intervention control group. Reading intervention had a small impact on math applied problem solving (b*= 0.10). In addition, using instrumental variable estimation (IVE), we found that activating word-level reading skills through reading intervention impacted math fluency (b* = 0.44-0.50). These findings provide preliminary support of a causal mechanism of language in math fluency resulting from word- level reading ability.