Abstract Effect of grain boundaries on the magnetic field distributions in YBCO was studied by use of magneto-optical (MO) imaging technique. The grain boundary was artificially introduced with a multi-seeded melt-growth process. The current distribution in the sample was obtained from the MO image using an inversion scheme based on the inverse-matrix method. With this technique, we could obtain intergranular and intragranular critical current densities. Large depression of the intergranular critical current was observed in the sample with large misorientation angles.