The purpose of this study was to investigate thermal environmental factors, thermal clothing properties, and thermal sensation of the office workers in four selected office buildings in Seoul, and to determine the effect of thermal environmental factors and clothing insulation to the thermal sensation of the subjects. The subjects selected from each office were 5 males and 5 females at a time. Thermal environmental factors(DBT, GT, RH, MRT, $ET^{ast}$) and clothing variables such as clothing weight per body surface $area(g/m^2)$ and estimated clothing insulation values(clo) were significantly different among each seasons(p<0,001). Means of $ET^{ast}$ and estimated clothing insulation values of each season were as follows; Winter; $20.84^{circ}C$ $ET^{ast}$ with 0.72 clo for male and 0.79 clo for female Spring and fall; $23.65^{circ}C$ $ET^{ast}$ with 0.59 clo for male and 0.68 clo for female Summer; $26.00^{circ}C$ $ET^{ast}$ with 0.47 clo for male and 0.53 clo for female. In comparison these data with ASHRAE Standard, the subjects were predicted to feel comfort-able in spring and fall, and slightly hot in summer and slightly cold in winter because of high and low clo respectively. But the result of this survey showed more than $80\%$ of the occupants were thermally comfortable at a given environmental temperature and clo.