This study reported the prevalence of hypertension and related factors among children at first-grade in elementary school children in Taipei. We measured height, weight, neck circumference (NC), waist circumference (WC), systolic blood pressure, and diastolic blood pressure in a regular health examination among children at first grade in 5 elementary schools in Taipei in 2007. According to the criteria of hypertension from American Pediatric Association and the recommendation of anthropometric measures from Department of Health in Taiwan, we determined hypertension, overweight, and obesity, high NC, and high WC. Overall, there were 2,344 eligible school children enrolled in this study. The prevalence of elevated BP was as high as 19.0% in these children. Compared with girls, boys had higher prevalence of hypertension (22.1% vs. 15.7%, p = 0.0002). Compared with reference group, children with high NC (41.9% vs. 17.5%, p < 0.0001) or high WC (45.9% vs. 17.4%, p < 0.0001) had higher prevalence of hypertension. The prevalence of hypertension was higher in obese children than in children with normal weight (37.6% vs. 15.8%, p < 0.0001). In the multivariate logistic regression, boys had higher risk of hypertension than girls (OR = 1.41, 95%CI = 1.13–1.75). The OR of hypertension in children with high WC was higher than children with normal WC (OR = 1.99, 95%CI = 1.18–3.37). Obese children (OR = 1.83, 95%CI = 1.14–2.92) or overweight children (OR = 1.99, 95%CI = 1.48–2.68) had higher risk of hypertension compared with children at normal weight. This study found that overweight or obesity was associated with hypertension. We suggested that the measurement of blood pressure should be included in regular examination among school children.