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The physical properties of the municipal solid waste (MSW) in Kahrizak Landfill (Tehran, Iran) and its changes due to aging were investigated in this research. A study of the components of the fresh MSW in this landfill showed that more than 60% of it was made from the wastes of foods, fruits, vegetables and organic materials. Next to that, paper/cardboard and plastics, with contributions of 14% and 11%, comprised the greatest parts of the waste materials. Meanwhile, the results obtained from these studies revealed that the contribution of the organic part has been decreased during the last two decade by about 20% while the plastics and paper/cardboard contribution has been increased by the same amount. In order to investigate the effect of aging on the physical properties of MSW, waste samples of 5.5, 14 and 21 years of age were obtained by excavating the aged waste burial regions of this landfill. A study of the changes in the composition of waste materials through aging also revealed that the portion of paste was decreased from 25% to 40% due to the decomposition process, while the contribution of plastics and fabrics was increased up to 200%. Particle size became finer with the mean size being reduced from 70 mm in the fresh wastes to 20 mm in 21-year-old wastes due to the decomposition process. The moisture content of the fresh waste samples was reported to be more than 150%, which was considerably larger than that of other existing landfills. Along with the increase in the age of the waste samples, the moisture content was decreased by as much as one third of the initial value. Furthermore, since the waste mass became more homogeneous by age, the variation of the moisture content was reduced. The organic content of the 14-year-old waste was found to be 20%, which was less than 0.3 of the initial value. Moreover, the variation of the organic content in the waste samples was directly related to the moisture content of the samples with both parameters being reduced to less than one third of the initial value in the older samples. Investigation of the moisture content and the organic content of the aged samples showed that the burial location had a significant effect on the trend of variations. The average density of the fresh waste was measured to be 3.5 and 7.3 kN/m3 after production and burial, respectively. It was found that the average density of the fresh waste grew to about 12kN/m3 as the age was increased.