[Background] The detection limit of manmade radionuclide in airborne monitoring is an important specification to measure detectable ability of airborne monitoring for nuclear and/or radiation. [Purpose] This study aims to find effect factors of detection limits of manmade radionuclide in airborne monitoring. [Methods] Based on the airborne monitoring data for the airborne monitoring system with NaI（Tl） detectors of the large volume（downward-looking for 50 341 cm3/upward-looking for 12 585 cm3）, Y-12 and Y-5 model wing-fixed aircraft, different altitude, and areas with the different radiation background levels, research diversification of detection limit of manmade radionuclide 137 Cs and 60 Co. [Results] Detection limits of manmade radionuclides decrease with the reducing of airborne altitude and flight speed. On the same airborne altitude, detection limits are corresponding to largen with the raise of radiation background level. Detection limits of manmade radionuclides are of the minimum on the waters. Detection limits are opposite to largen with the reducing of gamma-ray energy of manmade radionuclides. [Conclusion] Research provides important basis on how to control factors effectively and reduce detection limits of manmade radionuclides.