True random number generators (TRNGs) play a fundamental role in cryptographic systems. This brief presents a new and efficient method to generate true random numbers on field programmable gate array (FPGA) by utilizing the random jitter of free-running oscillators as a source of randomness. The free-running oscillator rings incorporate programmable delay lines (PDLs) to generate large variation of the oscillations and to introduce jitter in the generated ring oscillators clocks. The main advantage of the proposed TRNG utilizing PDLs is to reduce correlation between several equal length oscillator rings, and thus improve the randomness qualities. In addition, a Von Neumann corrector as post-processor is employed to remove any bias in the output bit sequence. The validation of the proposed approach is demonstrated on Xilinx Spartan-3A FPGAS. The proposed TRNG occupies 528 slices, achieves 6 Mb/s throughput with 0.999 per bit entropy rate, and passes all the national institute of standards and technology (NIST) statistical tests. © 2004-2012 IEEE.