The sodium flow resistance in sodium-cooled fast reactor cores experiencing natural circulation conditions was measured for wire-wrapped 19- and 37-pin bundles using low-velocity water flows with Re <1000 and Re <750, respectively. The measurements were compared with predictions of existing wire-wrapped bundle friction factor correlations. The results show that the existing correlations usually underestimate the friction factors in the transition flow regime particularly for those with small transition Reynolds numbers from laminar to turbulent flow. The reason for the underestimation is that the transition Reynolds numbers observed in this study were much smaller than the predictions of all the existing correlations, and as a result, the transition flow at the small Reynolds number was treated as laminar or quasi-laminar flow by the correlations. In addition, the quasi turbulence in the early stage of transition flow should have a significant influence on flow resistance.