The objective of this study is to investigate the initial flame propagation characteristics of turbulent flame in an engine cylinder through time-series analysis of radical emissions. A spark plug with optical fiber was developed in this study. The plug sensor is M12 type that makes it possible to mount in practical engine. The spark plug sensor can detect radical emissions in time-resolved spectra through time-series spectroscopic measurement. In this spectra, some kinds of radical emissions such as OH*(306nm), CH*(431nm) and C2*(517nm) based on principle of chemiluminescence are observed. In this study, the spark plug sensor was applied to both compression-expansion machine (CEM) and practical engine. As a result of CEM with bottom viewed high-speed camera, three kinds of spectra could be detected. At first, after the spark, there were radical emissions of the CH* and C2* in case that a spark plug sensor detected the flame front, and then the CO-O recombination emission was observed in the burned gas. As a result of practical engine, an hour durable experiment for spark plug sensor was implemented to confirm its durability by practical engine with high engine speed at wide-opened throttle condition. The cycle-to-cycle variation in lean burn condition was investigated using CH* as a marker of flame front. The history of CH* obtained 5.2kHz time-resolved spectra were compared with the history of mass fraction burned (MFB) analyzed from pressure history. In high engine speed at wide-open throttle condition, intensity ratio of CH* and C2* could be evaluated with air/fuel ratio nearby the spark plug.