Body mass index (BMI) is widely used to assess the prevalence of childhood obesity in populations and its relationship with blood pressure has been observed. However, no studies on the distribution of fat mass index (FMI) and fat-free mass index (FFMI) and their relationship with blood pressure have been reported.The present study examined the distribution of BMI, FMI, FFMI and the relationship with blood pressure among children aged 7-12 years in Shandong, China.A total of 4326 children (2165 boys and 2161 girls) aged 7-12 years participated in this study. Height, weight, skinfold thickness (SFT) and blood pressure (BP) of all subjects were measured, body fat percentages (BF%) were calculated by regression equation, fat mass index (FMI) and fat-free mass index (FFMI) were calculated according to the following expressions: FMI (kg/m(2)) = BF% × weight/height(2) and FFMI (kg/m(2)) = (weight - BF% × weight)/height(2).SBP and DBP were positively correlated with FMI, FFMI and BMI in both boys and girls, the strongest correlation observed for BP was with FMI. The prevalence of relatively high BP increased with FMI percentiles, this trend being especially obvious in upper percentiles of FMI. The prevalence of relatively high BP increased from 3.74% (boys) and 7.48% (girls) in the < 5th FMI percentile group to 56.19% (boys) and 50.94% (girls) in the ≥ 95th FMI percentile group, an increase of 14.0- and 5.8-times.There is a strong positive relationship between FMI and BP in children; the correlation between BP and FMI was stronger than with FFMI and BMI.