OBJECTIVE:Accurate measuring of blood pressure is a vital step in both clinical and para-clinical settings. The aims of the present study were to evaluate the validity and inter-observer reliability of measured blood pressures by two trained observers and one expert supervisor in the PERSIAN Guilan cohort study (PGCS). PARTICIPANTS AND METHODS:In a quasi-experimental study, two trained observers and one expert supervisor measured systolic and diastolic blood pressures (SBP and DBP) in 85 included participants. All measurements were done using Riester mercury sphygmomanometer as duplicate for each people. RESULTS:Lack of validity in the total SBP (P = 0.017), DBP in age <50 years (P = 0.039), and DBP in BMI >25 (P=0.019) of first observer and total SBP (P = 0.045), SBP of male (P = 0.019), both SBP and DBP in age >50 years (P = 0.034, P = 0.012) and DBP in BMI <25 (P = 0.001) of second observer were seen. In addition, total inter-rater reliability was found as 12.2% and 27.2% in SBP and DBP, respectively. Age, sex and BMI categorized inter-observer reliability were not more than 15% in SBP of BMI <25 kg/m and 31.6% in DBP of female. The final inter-observer agreement after educational course was higher than 0.7 totally and in all categorical evaluations. CONCLUSION:Based on lack of validity in some conditions and low level of reliability, education of all observers to measure both SBP and DBP accurately is needed. This is more necessary to done before performing the high population surveys.