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We evaluated the doping effect of single walled carbon nanotubes (SWCNT) on the phase formation, microstructure, critical current density (Jc), and upper critical field (Hc2) of MgB2 superconductor. All samples were sintered at temperatures ranging from 650 to 1000degC for 30 min. A typical high sintering temperature (900degC) sample was composed of well-consolidated MgB2 grain of 100 to 200 nm and nanosized particles, resulting in enhancement of Jc and Hc2. This indicates that lattice defects and microstructural change have occurred as a result of carbon (C) coming from SWCNT. On the other hand, a sample sintered at low temperature (700degC) contained relatively large grains. Specifically, SWCNT doped MgB2 wire sintered at 900degC exhibited excellent Jc > 104 Acm-2 up to 8 T at 4.2 K. This result indicates that flux pinning for sample produced at a high sintering temperature are enhanced by SWCNT doping. SWCNT is one of the promising C sources for MgB2 superconductor with excellent Jc.