  Blue compact galaxies (BCGs) are compact galaxies that are dominated by
intense star formation. Comparing the observational properties with the
predictions of stellar population synthesis model, we have analyzed the nuclear
stellar population and emission line spectra in a sample of 10 BCGs. The
results indicate that the continuum flux fractions at 5870{\AA} due to old
stellar components and young stellar components are both important. The
contribution from intermediate age components is different in different
galaxies. Our results suggest that BCGs are old galaxies, in which star
formation occurs in short intense burst separated by long quiescent phases. We
have also derived the internal reddening for the stellar population by
population synthesis method, and the internal reddening for the emitting gas
clouds by the Balmer line ratio. The former is significantly smaller than the
latter for BCGs. A model of clumpy foreground dust, with different covering
factors for the gas and stars, can explain the difference. Combining the
internal reddening value and the stellar population, we have decreased the
effecting from the internal reddening and underlying stellar absorption, and
accurately measured most emission lines for each BCGs. Using these emission
lines, we have attempted to identify the ionizing mechanism of BCGs. The
ionizing mechanism for these emission line regions of BCGs is typical of
photoionization by stars, characteristics of a low extinction HII regions.
