In this study, we developed 3D-UG with augmented reality (AR) to facilitate students while learning 3D geometry in authentic contexts. 3D-UG provided exploring activity with real-time measuring the authentic 3D objects in surrounding with AR. Students learn 3D geometry, measure 3D objects and calculate the volume or surface of the authentic 3D objects in their home during pandemic Covid-19 for one month. Participants were forty fifth-grade elementary schools’ students that divide into two groups, twenty students in the experimental group (EG) used our purpose 3D-UG and twenty students in the control group (CG) used ruler. The results found that students in EG have improved geometry ability and outperformed compared with CG. Students in EG has a better understanding of geometry concept than CG. In addition, students in EG made fewer mistakes in their formula input and calculation process. It is because students carefully complete the solution and 3D-UG has clear steps to accomplish the solution. Therefore, our 3D-UG with AR could facilitate students to learn geometry learning in their home during Covid-19, exploring 3D objects their surroundings, and experienced 3D object measurement with augmented reality to enhance their geometry ability.