The relation between the length of the magnetic core (l) and the giant magnetoimpedance(GMI) effect of solenoid with magnetic core of Fe36 Co36 Nb4Si4.8 B19.2 ribbon which annealed with current 32 A/mm2 for 10 min, has been investigated. The result shows that there is a sensitive relation between l and GMI. There is a linear relation between the l and the maximum of the GMI (Z/Z)max when l is smaller than the length of the solenoid l0. The linear correlation coefficient can be deduced from the electromagnetism theory. It should deviate from the linear relation when l is greater than l0. There is a maximum value of the (Z/Z)max when the l reaches the best value. There is a similar relation between the height of the tip like part of the GMI profile (Z/Z)T and l. There is a maximum value of the (Z/Z)T when the l is the best value. The tip like part of the GMI profile, which responds to the weak magnetic field sensitively, is influenced by the demagnetizing field. There is a negative exponential relation between the demagnetizing field and the length of the magnetic core l.