Optical fiber pressure sensors developed recently have merits of compact size, installation ease and relatively low cost. However, they are normally not suitable for IMEP measurement due to the insufficient measuring accuracy in low frequency range, which is vital for the measurement of indicated mean effective pressure (IMEP). On the other hand, it is confirmed that the IMEP is determined by the frequency components which are equivalent to, or twice greater than those of the engine speeds included in the indicated pressure wave form. The IMEP calculation method discussed in the following is capable of accurate calculation of IMEP, by checking the characteristics of the pressure sensor in advance and correcting the primary and secondary components of frequency properly. This paper describes the correction method and reports the measured results obtained in actual engine operations.