Transition metal dichalcogenide (TMD) monolayers attract great attention due to their specific structural, electronic and mechanical properties. The formation of their lateral heterostructures allows a new degree of flexibility in engineering electronic and optoelectronic dervices. However, the mechanical properties of the lateral heterostructures are rarely investigated. In this study, a comparative investigation on the mechanical characteristics of 1H, 1T′ and 1H/1T′ heterostructure phases of different TMD monolayers including molybdenum disulfide (MoS2) molybdenum diselenide (MoSe2), Tungsten disulfide (WS2), and Tungsten diselenide (WSe2) was conducted by means of density functional theory (DFT) calculations. Our results indicate that the impact of the lateral heterostructures has a relatively weak mechanical strength for all the TMD monolayers. The significant correlation between the mechanical properties of the TMD monolayers and their structural phases can be used to tune their stiffness of the materials. Our findings, therefore, suggest a novel strategy to manipulate the mechanical characteristics of TMDs by engineering their structural phases for their practical applications.