By using a premixed laminar burner, the effect of mixture component on laminar burning velocity($S_L$) was investigated. The following was made clear ; (1)As the humidity$(H_2O)$, $CO_2$ and Ar in mixture is increased, $S_L$ decreased in proportion to quantity of those dilution gases. (2) The heat reaction theory says that mean thermal conductivity $(lambda_m)$, specific heat $(C_{pm})$ of mixture and adiabatic flame temperatures $(T_b)$ affect $S_L$. As a result of theoretical analysis, the effect of $lambda_m;and;C_{pm}$ on $S_L$ is less than 1/25 of the effect of $T_b$, so the effect of $lambda_m;and;C_{pm}$ can be ignored. (3) From experimental results, it was confirmed that $ln(S_L)$ is proportional to $(1/T_b)$, that is, the effect of $H_2O$ on $S_L$ is mainly caused by changes of $T_b$. This conclusion was verified by the fact increases of $H_2O,;CO_2$ and Ar decrease the intensity of radiation typical $C_2$, CH, and OH in the same manner.