Through-the-thickness cracks with the depth of about 2.5 mm were introduced on the flexural specimens of alumina. Borosilicate glass was then penetrated into the cracks at 1500 degrees C to repair them. Strength testing indicated that the cracks, once repaired, became even stronger than mother material alumina. The reason of the strengthening was discussed in terms of residual stress and the rebonding of alumina grains on crack surfaces. This investigation implies that cracks in alumina can be repaired by simply penetrating glass into them.