Ammonia is expected to be not only as a hydrogen energy carrier but also as a carbon-free fuel. However, few studies of ammonia flame have been studied because of its lower combustion intensities. In this study, laminar burning velocity and Markstein length of ammonia/air premixed flames were experimentally evaluated from spherically propagating laminar flames. Equivalence ratios were varied from 0.7 to 1.3 and initial mixture pressure was set to 0.1 MPa. Spherically propagating laminar flames in a constant volume combustion chamber were observed by Schlieren photography with a high-speed camera. As a result, unstretched laminar burning velocities obtained in this study were slower than 8 cm/s for all examined equivalence ratios. The Markstein length increased with the increase in equivalence ratio. At the conditions of equivalence ratio of 0.8 and 0.9, Markstein lengths were negative. The numerical simulation using CHEMKIN-PRO with detailed reaction mechanisms were also performed. The reaction mechanisms must be improved in order to predict the unstretched laminar burning velocity of ammonia/air premixed flames.