Thin film coatings have a very important role in semiconductor technologies, optical devices, spectral selective coatings and other Micro/nano electromechanical applications. Due to wave interference, by adding a thin film coating to a surface, radiative properties will be completely different. Radiative properties of a thin film multilayer structure depends on many factors. Material, Thickness and Temperature of layers are the most important factors. In current project radiative properties of thin film multilayers are calculated using electromagnetic methods and material and thickness of each layer are optimized for desired applications, using Simulated Annealing and Genetic Algorithm in MATLAB software. Passive cooling is one of the investigated applications. If humidity is not too high, the atmosphere will act as a heat-sink in 8-13 µm waveband, which is called "atmospheric window". If energy transfer is limited in this waveband, by using an spectral selective coating, cooling can be possible without any energy consumption. Applications are food and medicine storage, roof cooling in dry climates and condensing atmospheric water. Maximum Cooling Power is accessible by placing an object under bare sky at night, But a convective shield is required to reduce convective heat transfer to ambient and increase temperature drop. Due to high Solar irradiance during day, passive cooling can be very difficult under direct sunlight. Preparing a proper convective shield for passive cooling during day is still an unsolved challenge in material science. Considering Cooling Power as objective function in Optimization, 15 different thin film multilayer structures are introduced for passive cooling during day in this project. Based on calculations a small temperature drop about 2-3 ?C, can be possible under direct sunlight by using some of introduced thin film multilayers. Temperature drop at night can be increased up to 105% by using these structures. 12 optimized thin film multilayers are also introduced for passive cooling at night. Based on calculations temperature drop can be increased up to 120% by using introduced structures. Using a layer of KBr or NaF (with hight solubility in water) coated from both sides with thin films is introduced for the first time as a very proper convective shield for passive cooling at night, which will result in 123% increase in temperature drop. Preparing a suitable Heat Mirror is another investigated application in this project. Heat Mirror is an special kind of glass with high Transmittance in visible range and high Reflectance in infra-red region, to prevent radiative heat transfer with ambient and decrease heat loss as much as possible. 6 optimized thin film multilayer structures are introduced to modify radiative properties of a single layer of SiO 2 for use as a Heat Mirror. Reflectance of SiO 2 in infra-red region can be increased up to 415% by using proper thin film coatings. BaTiO 3 is also introduced as a proper Heat Mirror for the first time. 20 optimized structures are also introduced, with maximum absorptance in solar region. Such high absorptance structures can be used in Solar Collectors and Solar Cells. Simillar structures are also introduced with maximum transmittance and maximum reflectance in solar region. Keywords: Heat Transfer, Thin Films, Nano-scale Radiative Heat Transfer, Thermal Radiative Properties, Passive Cooling, Heat Mirrors, Optimization