This research studied the mechanical properties and thermal conductivity of the interlocking block containing biomass ash. Portland cement to sandy soil and to laterite soil ratio of 1: 5.8: 1.6 with the replacement levels of sandy soil with biomass ash of 0, 20, 40 and 60 % by weight of sandy soil were used in this study. The specimens at the age of 28 days were tested for compressive strength, unit weight, water absorption and thermal conductivity. The test results showed that the increasing of biomass ash resulted in the decreasing of compressive strength, unit weight, and thermal conductivity and the increasing of water absorption. All of these specimens passed the standard requirement as per Thai Industrial Standard 602/2547 for non-load bearing type of interlocking block and thus can be used as the commercial interlocking block. Moreover, these interlocking blocks had low thermal conductivity values which are suitable for use as the internal wall for the building insulation.