The work is devoted to estimating the measurement uncertainty of sensitivity to gamma radiation of Cs-137 rectangular detectors based on polystyrene scintillators (PSs). The detectors are included in the radiation portal monitors as the sensitive elements. The detectors were manufactured in the Institute for Scintillation Materials of the National Academy of Sciences of Ukraine (ISMA NASU). They consisted of PSs with sizes from 500x300x50 mm (7500 cm3) to 1500x500x50 mm (37500 cm(3)) connected to a photomultiplier "Hamamatsu" R1307. Protective lead coating was absent. The sources of detectors uncertainty measurement sensitivity to gamma radiation of Cs-137 were determined and the mathematical model for their evaluation was made. The effect of the detectors sizes and the distance h from detector to ionizing radiation source on the uncertainty value was studied. It is shown that with the increase in the detector size, the uncertainty contribution of the detectors counting rate increases by 30 % (from 0,62 to 0,81 %). At this, the contribution of the background counting rate uncertainty increases by a factor of 5 (from 0,05 to 0,24 %). The change of the distance h from 10 to 50 cm increases the detectors counting rate uncertainty contribution by a factor of 2 (from 0,33 to 0,64 %) and the background count rate by a factor of 10 (from 0,04 to 0,35 %). Calculated relative value of the combined uncertainty by type B evaluation was determined by additional influencing factors. For all the investigated detectors it varied in the range from 10,2 to 10,9 %. The relative value of the expanded uncertainty had the range from 20,3 to 21,8 %. A comparison of PS detectors sensitivity values and expanded uncertainty of its measurement determined in ISMA NASU and other companies showed the similarity of measurement procedures.
