An Al-1.0 mass%Mg₂Si alloy was aged at 473 K for 36 ks. Using the same sample, resistivity (ρ) measurements, tensile tests and TEM observations were carried out. The volume fraction (V_fρ) of precipitates was estimated from the decrement in resistivity due to aging. Length (λ), diameter (d) and number density (N_P) per unit volume of the needle-shaped precipitates were measured on TEM micrographs. Perhaps because of initiation of overaging in the present aging condition, λ and N_P showed a wide scatter and differed significantly in two equivalent samples, independently heat treated and tested. However, proof stress and V_fρ in the two runs well coincided with each other and the volume fraction estimated from TEM micrographs (V_{f TEM}) matched within a factor of 2 for the two runs.