In this paper, a new methodology for the field measurement of thermal resistance of building envelope is proposed. A plane heater such as electric blanket is applied to heat one side of the test wall and heat flux transducers are fixed on the other side of wall to measure the heat flux through the measurement section. This could reduce the equipment size significantly and make the installation more easily compared to the traditional hot-box method but still can create the enough temperature difference and one-direction heat flux to calculate the thermal resistance of wall in a relatively short period. For one of the most common type of wall used in southern China, extensive In-situ measurements and numerical simulations were carried out to identify the influence of plane heater size to the accuracy of thermal resistance measurement. The evaluated thermal resistance values can be considered as or at least close to the true value when the heating sizes are larger than 1mx1m(3.3 ft x3.3 ft), but these values are always overestimated and deviate rapidly from the true value when heating sizes are less than 1mx1m(3.3 ft x3.3 ft). However, it is possible to amend those overestimated values by the curve developed in this paper.