This paper presents an implementable distributed load flow calculation method for online analysis, for power systems having multiple, geographically separated areas. The system is partitioned into multiple areas, with each area having its own control centre. Adjacent areas share a tie-line and the power flowing in these tie-lines are expected to be known to the control centres concerned before the analysis is started. Since each area may be governed by different companies, it is justified to expect that the power that is traded through a tie-line would be known to the control centres sharing the line. The proposed method uses the fast decoupled load flow approach and it suitably adds elements to the B' and B matrix to find out the state variables of the boundary buses. Results on the IEEE 14 and 30 bus systems show that the distributed analysis converges as fast as the centralized ones. © 2008 IEEE.