Video security has become more and more important with the widespread use of videos. The encryption algorithm developed to protect text data may not be suitable to encrypt videos because of large data size and high real-time demand. Light weight algorithms, especially selective encryption algorithms, are attractive. However, existing algorithms can’t meet requirements in cryptography security, encryption efficiency and compression efficiency at the same time. We propose a fast selective encryption algorithm in this paper. Our algorithm selects data randomly instead of selecting key information. This is achieved by generating several pseudo-random sequences using RC4 with separate keys. The sequences are labeled with ‘0’ and ‘1’, and whether one bit in video data is encrypted is determined by the corresponding bit in the sequences. Our algorithm is at least as safe as naive algorithm for ciphertext-only attack, knownplaintext attack and chosen-plaintext attack. On the other hand, our computational cost is less than 7 percent compared with naive algorithm. Furthermore, our algorithm doesn’t enlarge video size and keeps video codec and video format compliant.