We fabricated in situ powder-in-tube-processed MgB₂/Fe tapes using sub-micrometer Mg powder prepared by applying a thermal-plasma method or using aromatic hydrocarbons as additives, and investigated the superconducting properties. We found that the use of sub-micrometer Mg powder or aromatic hydrocarbons as additives was very effective for increasing the J_c value. The transport J_c value of 10mol% SiC-added tapes fabricated with sub-micrometer Mg powder reached 250A/mm² at 4.2K and 10T. This value was about twofold higher than that of the 10mol% SiC-added tapes fabricated with commercial Mg powder. The transport J_c value of 20mol% benzene-added tapes reached 130A/mm² at 4.2 K and 10T. This value was almost comparable to that of the 10mol% SiC-added tapes. Microstructural analyses suggest that this J_c enhancement is due to both the substitution of carbon for boron in MgB₂ and the smaller MgB₂ grain size.