This paper describes optimal thinning of a Elliptical Antenna Arrays (EA) of uniformly excited isotropic antennas which can generate directive beam with minimum relative Side Lobe Level (SLL). The Improved Particle Swarm Optimization (IPSO) method, which represents a new approach for optimization problems in electromagnetic, is used in the optimization process. The IPSO is used to determine an optimal set of ‘ON-OFF’ elements that provide a radiation pattern with maximum SLL reduction. Optimization is done without prefixing the value of First Null Beam Width (FNBW). The variation of SLL with eccentricity of thinned array is also reported. Simulation results show that the number of array elements can be reduced by more than 40% of the total number of elements in the array with a simultaneous reduction in SLL significantly.