In this paper, a novel quasi-isothermal method of measuring sample's specific heat capacity in temperature modulated differential scanning calorimetry (TMDSC) has been studied. With the strict temperature variation rule of plate-like sample in TMDSC model, the expression of the sample's surface temperature lag in quasi-isothermal state has been obtained. With this temperature lag rule, sample's specific heat capacity and its thermal conductivity can be determined at the same time by quasi-isothermal experiment of TMDSC. It has been pointed out here that the obtained heat capacity value of the sample with the quasi-isothermal method is only the average value within the measured temperature interval, and it is true that the bigger the modulated amplitude, the smother the value of heat capacity obtained. In the situation that the specific heat capacity of sample is approximately constant within the measured temperature interval, the quasi-isothermal method of TMDSC is better. If within the measured temperature interval the specific heat capacity of sample is apparently the function of temperature, the best tool to measure the sample's specific beat capacity is traditional DSC.