The fracture toughness of the interface, G a , of the self-healed joints of poly(ethylene) (PE) was measured using the wedge method. Samples of PE modified by mixing with three additives (branched low-molecular weight PE, a graphite filler, and polypropylene oil) were investigated. The development of the strength of partially healed joints formed by several hours of contact at a welding temperature of 105°C can be represented in all cases by the linear dependence of the G a parameter on the square root of time, in accordance with the diffusion mechanism of the interface formation. The presence of the additive in samples was found to enhance the fracture toughness of a joint for a given welding time. In the graphite composites, an induction period of welding was observed. In contrast, an instant nonzero strength occurred in joints of PE with PP oil samples. The results confirmed that the concept of the chain entanglement control of fracture toughness developed originally for the glassy polymers is well transferable to the semicrystalline PE. However, additional mechanisms due to the crystallization of PE upon cooling are also effective in the development of the joint strength. These mechanisms denoted as cocrystallization, transcrystallization, local crystallization, mechanical interlocking, etc., are substantially affected by the concentration of additives.