This paper investigated the influence of assumptions in soot volume fr action measurement by thermocouple particle densitometry (TPD) method in laminar nonpremixed ethylene/air diffusion flame. TPD method is based on an extension o f the theoretical treatment of Eisner and Rosner, who assumed that the particle deposition to a cold surface immersed in a flame is dominated by thermophoresis. They developed a formulation for the soot particle mass flux to thermocouple be ad to infer soot volume fraction by inserting the thermocouple to flame rapidly. However, in their formulations, some important parameters were neglected, such as the radiation from soot to thermocouple bead. We added the parameter into the formulation and analyzed its impact. The results show that the ignored paramete r had significant influence for the calculation of thermocouple bead emissivity and thermophoresis is not the dominant mechanism during the initial stage of soo t particles deposition to bead, whose real causes should be studied further. This paper investigated the influence of assumptions in soot volume fr action measurement by thermocouple particle densitometry (TPD) method in laminar nonpreixed ethylene / air diffusion flame. TPD method is based on an extension of the theoretical treatment of Eisner and Rosner, who assumed that the They developed a formulation for the soot particle mass flux to thermocouple be ad to infer soot volume fraction by inserting the thermocouple to flame rapidly. However, in their formulations, some important parameters were neglected, such as the radiation from soot to thermocouple bead. We added the parameter into the formulation and analyzed its impact. The results show that the ignored paramete r had significant influence for the calculation of thermocouple bead emissivity and thermophoresis is not the dominant mechanism during the initial stage of soo t particles deposition to bead, whose real ca uses should be studied further.