Power Berries in Harvest Moon 64 are red fruits the player can find through doing different things. Some are easy to get, while some are more challenging. They are very valuable, and eating them will increase the player's stamina. There are a total of 10 in the game.
During the Winter season, smash the boulder on the frozen pond near the Fisherman's tent with a fully upgraded Hammer.
Till the land as much as possible, eventually you will find one. There is no trick to this one; it's just a matter of luck.
One can be found digging in the Mine during the Winter (usually around .
While the player is at full strength, throw an egg or vegetable into the Goddess Pond and wish for strength.
Go fishing off of the dock at the beach. Eventually, the player will catch a Power Berry.
Throw a Large Fish into the pond by the Fisherman's tent some time between 9:00 and 5:00, and the Kappa will give a Berry in return.
Give a marble to the Harvest Sprites. To get a marble, grow a minimum of 16 Pink Cat Mint Flowers (i.e. just under 2 full 3 x 3 flower beds. When all of the flowers are fully grown, Stu will come and ask for one. He doesn't have money, but he trades a marble for a flower.
Become friends with Basil, and he will give the player one when he returns in the Spring.