Determination of the in situ target strength (TS) versus fork length (FL) relationship is essential in order to apply hydroacoustic measurements to assess the abundance of fishery resources. Collections of in situ TS signals (using an EK500 sounder on board the R/V Ocean Research I) and corresponding fish size measurements for Scomber japonicus and S. australasicus (surveyed at the Nanfangao fishing port) were carried out concurrently in 1995. An optimal search algorithm using an Excel spreadsheet format was adopted to determine the in situ TS versus FL relationship. Results of the in situ TS-FL relationship obtained by the search algorithm, with a standard deviation of TS of 4 dB throughout all the size ranges, are as follows: TS (dB) = 29.5 x log (FL(cm)) – 90.0, and TS (dB) = 31.0 x log (FL(cm)) – 94.5 for S. japonicus and S. australasicus, respectively.