It is postulated that CBM slope is regarded as a good indicator of interventions teacher used. Although this assumption is important, there are few studies focusing on an unbiased estimate of CBM using empirical evidence. This study investigated that evidence of bias on CBM slope was found at the first graders. Participants in this research were 47 students from Grade 1 at elementary school. CBM math was used for estimating CBM slope. CBM math were administered weekly during 7 weeks. Multimodel inference was used for main analysis method. This study showed that CBM slope bias was not found for sex and SES. Instead, intercept bias was found for sex. Implication and limitation for this study are discussed.