The fuel assembly design with tightened fuel rod pitch gives a possibility of higher conversion ratio for Boiling Water Reactor. Pressure drop is one of the key design parameters to evaluate the possibility of BWR core with higher conversion ratio. The pressure drop tests for tight lattice rod bundle under steady conditions have been done under BWR fluid conditions. Two kind of a mini tight lattice bundle were used. One was a 7-rod test bundle with a hexagonal channel box. The other was a 14-rod test bundle with a rectangular channel box. The purpose of the 7-rod bundle tests was to survey the rod gap effect and the heating length effect on pressure drop. That of the 14-rod bundle test is to check the bundle shape and flow shape. The each pressure drop elements (head loss, friction loss, acceleration loss and spacer loss) were predicted based on the appropriate correlations for each item. The total pressure drop was obtained by sum of each pressure drop element. It was found from the compassion with data that the present prediction method has reasonable performance. The average and the standard deviation for the ratio of the predicted and the measured value is 1.05 and 9.7%.