The growth rate of a root is a useful indicator for the evaluation of nutrition, and for the presence of pollutants, allelochemicals, and herbicides. For its sensitive and real time measurement, we have developed a computer‐aided microscopic image analyzing system which enabled us to determination the growth rate of aRaphanus sativaL. root to a precision of 0.1 μm min‐1at every 15 seconds. A sample harvested at the 3rd day after germination, for instance, showed a 7.3 μm min‐1growth rate. Moreover, a transient increase in the growth rate which occurred just after the release from a cold stress was sensitively detected.