Natural circulation boiling flow resistance characteristics in a 3 x 3 rod bundle channel are experimentally studied in this paper. In order to achieve a stable two-phase natural circulation flow, the boiling flow phases are identified and measures such as decreasing the inlet fluid subcooling degree and increasing the inlet local resistance are adopted. The ONB during the experiment is determined by the wall temperature drop. The variation of pressure drop components, including the gravity pressure drop, the frictional pressure drop and the acceleration pressure drop, is given. For high mass quality conditions, the mass flow rate is observed declining with the increasing heat flux when the frictional pressure drop and the acceleration pressure drop grow faster than the gravity pressure drop reduction. A few homogeneous flow models and separated flow models for the two-phase frictional pressure drop are compared against experimental results. The homogeneous flow models can well predict the frictional pressure drops for low mass quality conditions and the Duckler model predictions are the most accurate with a mean relative error of 15.7%. In terms of the separated flow models, the Mishima and Sun models predictions are in good agreement with experimental results respectively for low and high mass quality conditions. Based on the Chisholm correlation, a new correlation is fitted by experimental results. The friction factors obtained in single-phase experiments are used to calculate the liquid phase frictional pressure drops. The comparison against experimental results and data from other literatures shows the new correlation can well predict the two-phase frictional pressure drops under given flow conditions. (C) 2018 Elsevier Ltd. All rights reserved.