MgB2 tapes were fabricated by the in situ powder-in-tube method using C2H4O3-treated B powder as a source material. It is found that the Jc values of MgB2 tapes are significantly improved by treating B powder with C2H4O3. At 4.2 K, the Jc of the C2H4O3-treated sample sintered at 800 °C reaches 5 × 103 A cm - 2 at 12 T. The C2H4O3-treated B improves not only the upper critical field through enhancing intraband scattering but also the flux pinning by increasing grain boundaries and C substitution effects. A larger amount of intragranular structural defects and grain boundaries were found in the C2H4O3-treated MgB2 matrix. Benefiting from the good crystallinity, the C2H4O3-treated tape sintered at 800 °C shows a bigger active area fraction and a higher Jc than the sample sintered at 700 °C. These results suggest that treating B powder with C2H4O3 is an effective way to enhance the critical current density of MgB2 superconductors.