It has been reported that doping with aromatic hydrocarbons in an in situ powder-intube fabrication process of MgB2 tapes is effective for increasing the critical current density (Jc) under magnetic fields. In this study, the mechanism of the Jc enhancement by doping with C4H4S was investigated by transmission electron microscopy (TEM). The C4H4S-doped MgB2 tape has a denser cross-sectional microstructure than that of a non-doped MgB2 tape. Electron diffraction and dark-field TEM observation indicated that the grains of MgB2 are well crystallized and slightly increase in size with the C4H4S doping. X-ray diffraction and two dimensional elemental mapping using characteristic x-rays suggested that C atoms substitute part of B atoms in MgB2 crystals. Unlike other kinds of impurity doping, such as SiC-doping, there were few nanosized inclusions formed by C4H4S-doping. Thus, it is concluded that the Jc enhancement with the C4H4S doping is due to (i) increasing supercurrent and magnetic-flux pinning at grain boundaries by the formation of dense and well-crystallized MgB2 matrix and (ii) increasing the upper critical field by the formation of Mg(B,C)2.