Spatial grid units are usually used for investigate urban human mobility. These units can easily lead to a modifiable area unit problem (MAUP) stemming from size variations between grid units. Current research on urban human mobility does not consider the MAUP or its influence on the quantitative analysis of human mobility. To address this problem, we used massive mobile phone tacking data to conduct a quantitative analysis of the effects of modifiable areal unit problem when dividing the urban space into different size of grid cell sand determined that intra-grid movements increase approximately at a linear pattern, and inter-grid human mobility decreases with grid sizes in a linear pattern and thus grid cell size affects spatial decisions about urban human mobility. Different sized grids deliver inconsistent results when extracting important locations from a human mobility network. When combined with the land use data, the grid cells containing residential and industrial land use types are the most affected. This paper discusses possible means to mitigate the uncertainty problems.