Tasmania eucalyptus nitens is one of the most important plantation hardwood species used for paper production. Forest growers and wood processing companies have recently considered it for the production of high quality sawlog. The high quality sawlog, however, can be produced from pruned plantation eucalyptus niten as the unpruned one contains several knots and cracks which lessen the quality of the log. Thus, it is vital for forest growers to deliver pruned log to wood processing companies. The pruned log, however, could not be discriminated from unpruned stems by harvester within the plantation plot due to self (natural) pruning process of unpruned tree. This leads to the delivery of the pruned log to the processors challenging. Although wood processors use large x-ray image machines during processing to optimise wood recovery, high costs are incurred from transporting poor quality, knotty timber following the harvest. In this paper, a 17 year old eucalyptus nitens has been considered for non-destructive evaluation. The aim is to investigate the effects of the defects including knots and cracks on the ultrasonic wave. 12 samples from different parts of trunk have been selected and conditioned at the forest moisture content of 120% (70% water content). The samples were scanned by ultrasonic waves at every 10 cm distance in longitudinal direction and at every 45 degree spacing in circumferential direction along the samples. Results show that there is a significant difference between recorded ultrasonic waveforms propagated through unpruned billets and pruned ones. The unpruned billets had a larger effect on ultrasonic waves while the waves are relatively steady when pruned billets are tested.