As a result of sintering the powder workpieces large part of the separated free energy leads to higher density of sintered body. This in turn is accompanied by a change in linear dimensions of workpieces. These linear changes in addition to the alloying elements in iron matrices largely depend on the technological parameters of the sintering process – temperature, duration, protective atmosphere and others. This study monitored the impact of the type of protective atmosphere and duration of sintering on the size change of powder workpieces of iron powder ASC 100.29 alloyed 0,15 ÷ 0,60% P. Sintering is conducted at 1150ºC a duration of 15 ÷ 90min in two protective environments – endothermic gas and dissociated ammonia. Presented are graphical relationships of the relative change in the diametric dimensions of the workpieces in dependence on the concentration of phosphorus in them, the type of the protective atmosphere and the duration of sintering.