This paper presents a new WiFi microstrip antenna in 2.4 GHz band by using the differential evolution (DE). The antenna is small with less than a quarter–wavelength. The geometric structure is formed by a square patch grounded with a substrate and cut by two adjacent rectangles from an edge, and a microstrip feed line printed on the other side of the grounded substrate. The structure is parameterised as a solution vector, and the requirements of the antenna are coded as objective and constraints. Then a constrained optimisation problem is formulated. Some optimised antennas are found, which meet the requirements in the simulation. Prototypes of an optimised antenna within a size of 25 � 25 � 1.6 mm� are fabricated. The measured performance matches the simulated one as a whole. One antenna was mounted to a wireless router and another one to a personal computer for indoor use. And we found they worked well. The fabricated antenna can be applied in the space limited within 25 � 25 � 1.6 mm�.