Production of microcellular foam (cell size 1-100 ?m and cell density above10 9 cell/cm 3 ) from low density Polyethylene (LDPE) to improve mechanical properties of foams, has been the aim of the present research. Although, the difficulty of preparation microcellular low density polyethylene foam due to its weak nucleation behavior, the possibility of making microcellular foam from this polymer, by adjusting the conditions (temperature, pressure, type and value components Foam) and using nano Calcium Carbonate (CaCO 3 ) as nucleating agent in value of 0.5-7 phr, were studied. Mixing operation was carried out with twin extruder. In order to better distribution of nanoparticles in the polymer matrix and also to prevent decomposition of chemical blowing agent (ADCA) and crosslinking agent (DCP) during mixing operations, for each foam combination (with variable percentages of nano calcium carbonate) two master batch A (containing blowing crosslinking agents) and master batch B (containing nano CaCO 3 ) with different operation conditions: 100 rpm with temperature range 60-125 °C for A and 250 rpm with temperature range 80-175 °C for B, were prepared and mixed together (combination C). Finally, Foaming process was done by compression moldings at 140, 160 and 180 °C. By comparison of X-ray diffraction pattern of nanocomposite samples, maximum nucleation (due to better distribution of nano-particles) was happened in sample with 5 phr nano CaCO 3 , which resulted the foam with cell size of 27 ?m and cell density of 8*10 8 cell/cm 3 . The results of studding the morphology of nanoparticles in polyethylene matrix by SEM, showed aggromulation with increasing amount of nanoparticles. With measuring compressive strength and thermal conductivity of foam samples, direct relationship between density and these properties was found. Also, compressive strength improved up to 370% just by adding 0.5 phr nano CaCO3. Keywords: Polyethylene Foam, microcellular foam, Nanocomposite foam, Nano CaCO 3 , WXRD