The phase structures and transition behaviors of a series of mesogen-jacketed liquid crystalline (LC) polymers, poly{2,5-bis[(4-methoxyphenyl)oxycarbonyl]styrenes} (PMPCS), with different molecular weights (MW) and narrow MW distributions were studied using differential scanning calorimetry, polarized light microscopy, Fourier transform infrared spectroscopy, and one- and two-dimensional wide-angle X-ray diffraction experiments. The LC phase structures of this series of PMPCS samples were found to be strongly MW dependent. The PMPCS samples were amorphous when the MW is lower than a critical MW of approximately 1.0 x 10(4) g/mol (an apparent MW, M-n(a), measured by gel permeation chromatography calibrated with the polystyrene standards). For the PMPCS samples with MWs higher than this critical value, the amorphous samples cast from solution developed into a LC phase above the glass transition temperature upon the first heating. In between 1.0 x 10(4) g/mol < Ma < 1.6 x 10(4) g/mol, a columnar nematic (Phi(N)) phase was stabilized. Above the M-n(a) = 1.6 x 10(4) g/mol, a hexatic columnar nematic (Phi(HN)) phase was observed. Within these two LC phases, the building blocks were cylindrical shaped, which was attributed to a cooperative assembly of the PMPCS backbone and its laterally attached mesogenic groups. The diameter of this cylindrical building block was in the vicinity of 1.6 nm as determined by WAXD experiments. All the LC phases were found to be stable up to the decomposition temperature of the PMPCS samples. The MW dependence of the LC phase diagram indicated that a critical aspect ratio (the ratio between the length and diameter of the cylinders) of the cylindrical building blocks must be required to stabilize these LC phases. On the basis of Flory's calculation, the critical aspect ratio should be 5.44, and this value corresponded to critical cylinder lengths of around 8-9 nm. Therefore, the lowest degree of polymerization which would stabilize the LC phases is similar to39-42 for this series of PMPCS samples.