A new antenna for a wristwatch phone is proposed. The proposed antenna is a one-wavelength rectangular loop along the cap of the wristwatch phone. The loop is fitted at the periphery of the openable cap connected to the wristwatch case by a hinge. In order to discuss the antenna gain, we define the two different conditions as follows. When the user opens the cap and talks over the wristwatch phone, his wrist is held in front of his face, which is called the "talk position" in this paper. When the user closes the cap and waits for a call, his wrist rests down at the side, which is called the "stand-by position. " We measured the radiation patterns and calculated the pattern averaging gain (PAG) of the proposed antenna for the two positions. In addition, we compared the proposed antenna with the other antennas: a planar inverted F antenna (PIFA) fixed on the strap and a normal mode helical antenna (NMHA) installed on the case. As a result, the PAG of the proposed antenna was about -5.5 dBd, which was the same as the PAG of the other antennas for the talk position. In the case of the stand-by position, the PAG of the proposed antenna was about -3 dBd, which was 7 dB higher than that of the other antennas. The gains of the proposed antenna reached the goal, which was the PAG of a whip antenna of a handy phone held near the head. The results demonstrate that the proposed antenna is suited to a wristwatch phone.