Abstract: According to GB/T 2951.21-2008, hot set test of a cable rubber insulation was carried out, and the uncertainty of the test results was evaluated. The results show that the main factor affecting the hot set test results is the measurement of the elongation of the rubber insulation. The tensile stress, oven temperature and laboratory ambient temperature also have certain influence on the test results. The hot set test results of the test rubber insulation are as follows:the percentage elongation under high temperature and loading condition y=15% with the expanded uncertainty U=4.2% when the including factor k=2; the permanent percentage elongation after unloading and cooling y'=5% with the expanded uncertainty U=4.4% when k=2.