Massive machine type communi-cation (mMTC) is one of the key application scenarios for the fifth generation mobile com-munication (5G). Grant-free (GF) transmis-sion can reduce the high signaling overhead in mMTC. Non-orthogonal multiple access (NMA) can support more users for mMTC than orthogonal frequency division multiple access (OFDMA). Applying GF transmission in NMA system becomes an active topic re-cently. The in-depth study on applying GF transmission in patt division multiple ac-cess (PDMA), a competitive candidate scheme of NMA, is investigated in this paper. The definition, latency and allocation of resource and transmission mechanism for GF-PDMA are discussed in detail. The link-level and system-level evaluations are provided to ver-ify the analysis. The analysis and simulation results demonstrate that the proposed GF-PD-MA has lower latency than grant based PDMA (GB-PDMA), possesses strong scalability to confront collision and provides almost 2.15 times gain over GF-OFDMA in terms of sup-porting the number of active users in the sys-tem.