In this study, radon background fluctuation effect on the measurement of 214Pb, 214Bi and 226Ra low level activity concentrations were investigated in ventilated laboratory. For this purpose, detectors which had 150% and 20% efficiencies were used. In spite of the air ventilation, radon background fluctuation effect on the low level activity measurement was observed. 20% efficiency detector measured activity discrepancies were 5.7% for 214Pb, 8.7% for 214Bi and 5.8% for 226Ra. 150% efficiency detector measured activity discrepancies were 13.8% for 214Pb, 11.5% for 214Bi and 12.7% for 226Ra.