Assessment literacy refers to the knowledge teachers have about definitions of testing and the employment of this knowledge to classroom practices. This study inspected the extent to which high school teachers of English, mathematics, science, and social studies utilize assessment literacy and instructional improvement practices in their classrooms and the probable relationship between these two factors. To this end, ninety-seven teachers from sixty high schools across Zanjan province in Iran were selected based on convenient sampling. The instrument used was a 32-item questionnaire designed by researchers and validated by experts. A confirmatory factor analysis (CFA) was also conducted in order to validate the factor structure of the questionnaire. The results showed a strong positive relationship between assessment literacy and instructional improvement practices in all four subject areas through correlations. This suggests that showing an understanding of assessment literacy results in a greater understanding of instructional improvement in the classroom. Moreover, a one-way ANOVA was performed in order to determine if there is a significant difference in the use of assessment literacy practices and instructional improvement among different subject areas. The data revealed no statistically significant differences between neither assessment literacy nor instructional improvement scores by subject area. The study can be beneficial to school districts and teachers since the review of existing teaching practices and an increase in ongoing professional development in assessment literacy theories and practices may improve instructional improvement in all subject areas.