Marangoni instabilities in a sessile droplet evaporating at constant contact angle mode on a heated substrate were investigated through a series of experiments. The quasi-steady state flower-like patterns of Benard-Marangoni instabilities were observed, which uniformly distributed along the azimuthal direction of droplet. With the receding of triple line, the cell number of the Benard-Marangoni convection decreases one by one following a square function of the evaporating time while the flower-like pattern maintains the quasi-steady state continuously. Different from the classical polygonal cells in flat liquid layer, the shape of cells in droplet is circular-arc. The critical Marangoni numbers for the incipience as well as those for the disappearance of the Benard-Marangoni patterns were determined experimentally. Besides numerical simulations were conducted and the detailed characteristics of the Benard-Marangoni instability patterns inside the droplet are explained thoroughly. (C) 2019 Elsevier Ltd. All rights reserved.