Phase transitions and structures of the monomer of side-chain liquid crystalline polyacetylene containing biphenyl mesogen, namely, 5-{[(4'-pentyloxy-4-biphenylyl)carbonyl]oxy}-1-pentyne (A3EO5) were investigated by differential scanning calorimetry (DSC), one dimensional (1D) wide angle X-ray diffraction (WAXD), and polarized light microscopy equipped with hot-stage (PLM-hot stage). DSC and 1D-WAXD results show that A3EO5 forms an enantiotropic liquid crystal and displays four transitions during the cooling and subsequent heating processes. Upon cooling fromisotropic phase to roomtemperature, the sample forms smectic A phase and then leads to smectic B phase with pseudo long-range orientation within the layer structure, further lowering the temperature, A3EO5 forms smectic E phase with orthorhombic packing within the smectic layer and enters crystal phase. PLM results show that the sample forms spherulitic texture, focal conic texture and concentric arcs texture during the cooling process.