This research applies the magnetic method to determine reinforcement length in bored in situ concrete piles by investigating the magnetic difference between the steel reinforcement and the concrete pile body. Preliminary tests indicate the Z-component curves from magnetic prospecting at the center, east and west boreholes show a deep gradient changing from local minima to local maxima, which corresponds to the depth of the lower end of the steel reinforcement. Z-component curves at the south and north boreholes also show the same pattern, but instead the local maxima corresponds to the lower end of steel reinforcement. When there is a change in the number of steel bars in the reinforcement along the depth, a characteristic “valley between two ridges” pattern appears in the curve, denoting the position of the change. This method also can be used to survey old steel piles or bridge and dam foundations where foundation configurations are not available.