Inner-wood decay is hard to be detected. This research focused on inner-wood decay detection for Mahogany and Albasia Wood. Inner-wood decay can increase the possibility for tree or wood structure to collapse. An instrument is needed to detect inner-wood decay. Nondestructive testing (NDT) is one of the analyze technique for evaluating characteristic of a material. In this research, NDT is use as the analyze technique for getting the wood condition. There is lots of method for NDT, in this research ultrasound method with through transmission technique is chosen. Through transmission technique observe the amount of ultrasound that is sent from transducer transmitter through the wood that is tested and received by the transducer receiver. The amount of ultrasound that received will be higher if there is no inner-wood decay in the tested wood. The instrument that has been designed and realized can detect and classified inner-wood decay. The percentage of success in detecting condition Mahogany Wood with good quality is 98%, poor quality is 80%, and mediocre quality is 74%. The percentage of success in detecting condition Albasia Wood with good quality is 94%, poor quality is 86%, and mediocre quality is 72%.