Thick films of YIG (yttrium iron garnet), which is well-known as magnetic material for microwave devices, were prepared on YSZ (yttrium stabilized zirconia) substrate by screen printing process. Y2.9Ca0.1Fe4.9Zr0.1O12 (Y3—xCaxFe5—xZrxO12, where x = 0.1) thick film exhibited good magnetic properties of saturation magnetization 4πMS = 1550 G and ferromagnetic resonance linewidth ΔH = 116 Oe. In order to enhance these magnetic properties of YIG thick films, pressure was applied to each thick film in a CIP (cold isostatic pressing) equipment. It has been concluded that CIP plays a role reducing the porosity of the thick films. We derived the relationship between ΔH value and porosity. Due to the effect of CIP on YIG thick films, 4πMS increased up to 1680 G and ΔH decreased to about 79 Oe with applied pressure.