Securing multimedia transmission has become a challenging issue due to the popularization of real-time multimedia applications such as video surveillance, satellite communication and web cams. However, previous security algorithms do not always guarantee a satisfactory degree of media quality and latency. In order to provide both security and media QoS, a viable security mechanism must consider three properties: processing time, compression rate and security level. In this paper, we propose a light-weight encryption scheme without loss of security and media QoS, called Secure Real-time Media Transmission (SRMT) using two block transpositions and a XOR operation. SRMT is studied with respect to MPEG-4, which is widely used in today's multimedia applications. Experimental results with various MPEG-4 movies show that the SRMT scheme achieves real-time transmission of encrypted media data without loss of security and media QoS. Though SRMT is conducted on uncompressed raw data, SRMT encrypts 3 times faster than the AES encryption of MPEG compressed data. Also, we show that manipulating key frames and a compression method can lessen increasing ratio of encrypted MPEG size, e.g., 70.5% improvement over an existing combination method of block transpositions and XOR operations.