We examined the effect of lipid phase composition on the crystallization, polymorphic transition and stability of solid lipid nanoparticle (SLN) suspensions. A series of fine-disperse oil-in-water emulsions was prepared at an elevated temperature (75 °C) from a lipid phase containing different amounts of a high melting lipid (tripalmitin) and a low melting lipid (fish oil). These emulsions were cooled to induce crystallization and form SLN suspensions. In the absence of fish oil, the SLN suspensions formed a gel after the emulsified tripalmitin crystallized, which was attributed to particle shape changes leading to aggregation and network formation. Light scattering and rheology measurements indicated that incorporation of fish oil into the lipid phase (≥10 wt %) increased the stability of SLN to aggregation. DSC measurements suggested that crystallization, melting, and polymorphic transitions of SLN were influenced by the amount of fish oil incorporated. The rate of α- to β-polymorphic transitions of tripalmitin increased with increasing fish oil content, and tripalmitin crystals formed appeared to be less ordered as evidence by a lower melting temperature. Results suggest that the phase behavior and morphology of tripalmitin crystals can be altered by mixing them with low melting lipids such as fish oil thereby improving the stability of SLN suspensions to particle aggregation and gelation. Moreover, results show that fish oil, rich in ω-3 polyunsaturated fatty acids, can be successfully incorporated into SLN suspensions.