There arm two kinds of rotational speed changes in running of internal combustion engines - rotational speed fluctuation within each working cycle and rotational speed change of successive cycles. Application of transient rotational speed nφ of each crank angle and average rotational speed nc of any working cycle can rationally demonstrate the change of the rotational speed of I.C.E..Conventional speed-measure instruments may not be used to measure nφ and nc. Therefore authors take work on this measuring technique. This paper introduces the measuring principle and instrument and analyses the accuracy of measure.The developed instrument has been used on 5 types of diesel engines, such as 6135, 6130Q, R4100, etc. Measured results at steady condition, low speed radomrunning, sudden loading, free acceleration and starting process are given in this paper.