ZnO is identified as one of the promising coating materials in optical fiber for ammonia gas sensors. In this work, three-dimensional ZnO nanoflowers are fabricated via a facile hydrothermal method and used as sensitive coating materials for optical fiber ammonia gas sensors. Results show that the sensor with ZnO nanoflowers exhibits a high sensitivity of 5.75 pm/(μg·L-1) for ammonia concentration ranging from 0 to 5460 μg·L-1, which is more than 2.6 times higher than that of the sensor with a coating of ZnO microspheres [2.2 pm/(μg·L-1)].