To estimate Net Primary Production (NPP), measurements of amounts of the growth and litter of fine roots are important. We here describe a new method of estimating fine root growth and mortality simultaneously in one experiment using a manipulated soil cylinder. The manipulated soil cylinder is similar to fine root ingrowth core, but differs in two aspects by using two experimental treatments. First treatment removes all particulate organic matter from the soil medium used to fill the cylinder by soil washing. All dead organic matter at the end of experiment originated from fine roots that grew and subsequently died during the experiment. Second treatment uses the soil medium similar to the first treatment, but known amount of dead organic matter is added. Also, the cylinder is covered with water-permeable but root impermeable cloth to prevent fine root ingrowth into the cylinder. It allows us to estimate decomposition rate of dead organic matter. Such information is sufficient to calculate fine root growth, mortality, and decomposition during period of the experiment. An example of applying this technique in estimating fine root dynamics in Chamaecyparis obtusa stand in central Japan is described.