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Phase-pure Aurivillius Bi5Ti3FeO15 nanoparticles were successfully prepared using a sol–gel method. Behaviours of crystallisation were characterised by XRD, DTA, FT-IR, Raman spectra and HRTEM. Mass spectrum was used to monitor the combustion products of all organics in the xerogel during calcination. An intermediate product, Bi2O2CO3, which blocks the generation of the pyrochlore phases, was confirmed by XRD patterns and IR spectra. Calculated by XRD data using the Debye–Scherrer method, the average grain size in powders calcinated at 500 °C was 17.7 nm and grew to 80.5 nm at 850 °C. The local symmetry connected to FeO6 octahedral was broken. The XRD refinement and XPS analyses inferred the valence increase of ferric ions. The maximum magnetisation of nanoparticles prepared at 850 °C was 4.2 emu/g. The field cooling (FC) and zero field cooling (ZFC) tests showed a wide range of the blocking temperature (T B) around 8–45 K answering for the lack of hump corresponding to the transition from superparamagnetism to blocking state.