A mean field theory is introduced to describe the nematic-isotropic phase transitions (NIT) in solutions of main-chain liquid crystalline polymers (MLCPs) which consist of rigid mesogens and spacers with various degrees of flexibility. The theory takes into account not only the nematic ordering of mesogens but also the partial ordering, or straightening, of spacer segments in the nematic phase. On the basis of the Onsager-type excluded volume interactions and the Maier-Saupe model for orientational dependent-attractive interactions between rigid segments, we derive the free energy for solutions of the MLCP. We find two different nematic phases: One is a weak nematic phase which is almost formed by the ordering of the mesogens. The other is a high nematic phase where the straightened segments on the spacers and the mesogens are highly ordered. The two different nematic phases and phase behaviors are discussed in the phase diagrams on the temperature-concentration plane.