The application of 180 milli-tesla (ml.t) on larvae of silkworm, Bombyx mori L. (Lepidoptera: Bombycidae) was investigated to assess the effect of its different exposure periods; 1, 2, 4 and 8 minutes on some biological traits and haemolymph biochemical constituents. The magnetization each of the third and the fourth larval instar was carried out once on the first day before first feeding. Exposing silkworm larvae during each of the third or the fourth instars to the tested magnetic field for eight minutes increased mature larval weight followed by four minutes magnetization during both instars. Control group and the larvae exposed to the magnetic field for eight minutes manifest the highest cocoon shell weight of the resulting cocoons. Silk ratios were significantly higher in two minutes exposure to the magnetic field during the fourth larval instar followed by one minute exposure during the third larval instar. Variation in the exposure periods of the magnetic field remarkably influenced the biochemical component of the haemolymph of mature silkworm larvae. Stimulatingly, Alanine amino transferase activity, total protein content and Amylase activity were highest in haemolymph of the larvae exposed to the tested magnetic field during the third or the fourth larval instar for eight minutes, while Aspartate amino transferase activity and total lipid content were found to be the highest in the haemolymph samples taken from larvae exposed to the same magnetic field during the third larval instar for four and eight minutes, respectively.