The article presents a study to determine the thermal transmittance for buildings different walls structure. Thermal transmittance is used especially in western European countries to indicate the rate of heat transfer for different components of buildings: walls, ceilings, floors, windows, doors, etc. The first part of the article includes several thermal transmittance introductive notions. The second part of the article defines the global heat transfer and thermal transmittance and their calculating relationships. In the third part of the article is presented a comparison of thermal transmittance values for two different wall structures analyzed by numerical calculation so as MC 001/2009 and by measurements taken using specialized equipment Testo 435.