Two methods of generating random sequences by several typical physical chaotic systems, including Chua's circuit, Lorenz system and Chen's system, were given. And the randomness properties of the random sequences generated by the chaos systems were analyzed by using NIST's STS randomness test suite. Six tests, which are the monobit test, the runs test, the discrete Fourier transform test, the approximate entropy test, and the cumulative sums test, were conducted to compare the pseudorandom properties of these chaotic sequences. The experiment results show that the randomness properties of the sequences generated by physical chaos are better than those by simulation chaos, and NIST's STS randomness test suite can distinguish the randomness of different sequences. Among three typical physical chaotic circuits, we recommend the circuit of Chen's system to generate random sequences.