A spindle structure for a pedal includes a pedal body having a through hole formed from one end thereof to another end thereof, two ends of the through hole respectively defined as a first end opening and a second end opening; and a spindle body assembled on the pedal body, the spindle body having an insert portion and a threaded portion respectively defined at two ends thereof, the insert portion inserted into the through hole of the pedal body, a tip portion of the insert portion defined as a positioning portion, a periphery of the positioning portion gradually widened toward the second end opening, the threaded portion exposed from the first end opening, the positioning portion exposed from the second end opening.