Multilevel optical data storage is an important technology to improve the recording density of optical disc without changing the optical parameters. One approach is multilevel amplitude modulation, which uses the variance of signal amplitude of length-fixed recorded marks to record information. Another one is multilevel runlength-limited (RLL) modulation, which records information in both the signal amplitude and the length of recorded marks, thus can improve the recording density more effectively than the first way. Multilevel RLL coding is a key technology in multilevel RLL modulation optical recording. In this paper, a new 8-ary (1，3) RLL code is designed with state splitting algorithm. The recording density of this code is as high as 3.0 bits/symbol, and the encoder/decoder is simple and easy to implement. The proposed code can be used for future high density multilevel RLL modulation optical disc systems.