Transparent -alumina ceramics are fabricated using spark plasma sintering. Paramagnetic defects related to the optical properties of the ceramics have been investigated using electron spin resonance (ESR) analyses. An isotropic ESR signal at g=2.003 (S=1/2) with a linewidth of 0.5mT is formed during sintering. The g=2.003 signal intensity has a weak correlation with the absorbance in the visible region but does not correlate with the real in-line transmission (RIT) at 650nm. An ESR signal with a fine structure attributed to Fe3+ was detected in both the -Al2O3 starting powder and the sintered ceramic samples. The degree of c-axis orientation of the grains has been determined using the Fe3+ signal intensity, which depends on the angle between the directions of the c-axis and the applied magnetic field. The ESR analysis indicated that the c-axis tends to be oriented in the direction of the sintering pressure. The degree of c-axis orientation was found to correlate with the RIT in highly densified ceramics.