This research aimed to describe the geography of Malayan dialect in Sambas and Mempawah River Flow Areas. This study was descriptive quantitative in nature using dialectometry formulation measurement and descriptive qualitative using in-depth top-down reconstruction. The data of research was collected using in-depth interview. The data was analyzed using synchronic comparative method for language mapping, phonologically or lexically. The data reconstruction was analyzed using diachronic comparative method with top-down reconstruction technique to find retention and innovation. The diachronically data analysis was carried out utilizing Isodore Dyen’s Proto Austronesia (PAN) (1970). The result of research was presented in a descriptive text about the geography of Malayan dialect in Sambas and Mempawah rivers’ flow areas. Language mapping carried out phonologically found 5 dialects, while the one lexically found 7 dialects. Two lexicon reconstructions were found: (1) ‘relict’ retention and (2) retention. Prefix retention only found innovation. The highest retention distribution in Sambas and Mempawah River Flow Area was in TP 5 (Karangan), while the lowest one was in TP 4 (Sambas). The highest innovation distribution was in TP 1 (Seluas) and TP 6 (Menjalin), while the lowest one was in TP 4 (Sambas). The highest conservative area was in Sambas and Mempawah River Flow Area in TP 5 (Karangan) and the highest innovative area was in TP 1 (Seluas) and TP 6 (Menjalin)