The most economical modulation format solution is probably direct modulation deploying uncooled Fabry- Perot (FP) lasers for low-cost fiber optic links. However, transmission performance using uncooled EP lasers can be significantly affected by longitudinal mode characteristics such as mode modulation (=mode missing) [ 11 and TE-TM mode switching (=modal instability). There for it is necessary to understand mode missing and transmission performance including lasing spectrum of the uncooled FP laser with missing modes. From the measurements of longitudinal mode spectra of 1.3μm InGaAsP/InP Fabry-Perot lasers, we can find that five lasers out of 25 lasers satisfy the missing mode condition. The measured spectra moved to longer wavelength because of the gain curve shifts toward longer wavelength with increasing temperature. Fig.l(a) shows the measured longitudinal mode spectra without missing modes. However, other lasers have both mode missing and modal instability in Fig. 1 (b) and Fig. 1 (c). We believe that modal instability is due to modal gain change by the temperature-induced stress.