Electronic properties of pregraphitic carbons are usually studied using a free electron gas and a free charge carrier mass models (this mass decreases if the cristallite diameter increases). The two dimensional gas model[1] yields an analytic formula (1) which parametrizes experimental magnetic susceptibility of carbons with the help of two numerical parameters T 0 and K 0 . However, since diamagnetic susceptibility increases with crystallite sizes in the range of 30 Å to 200 Å, the electronic levels cannot yet be assumed to form an energy continuum (band). A recent calculation of magnetic susceptibility [5] shows that London theory [6] is well adapted to explain this diamagnetism. In this paper, each carbon is described by graphitic ribbons without interlayer interactions, but forming a Grand Canonical Ensemble. In such a way the diamagnetic formulas related to the π electrons are expressed in (4) and (5). The numerical results of Ref. [5] show that the negative χ d term is proportional to the width ribbon L , and is almost independant of temperature. The positive χ p term is due to a splitting of the degenerated levels. These properties lead to the following simplified model: in the vicinity of the energy gap δE , the energy levels are assumed to be equidistant when no magnetic field is applied and every term a i is assumed to be equal to a unit quantity a (Fig. 1). In the case where the graphitic ribbon width L is in the range of 30 Å to 100 Å and the temperature T in the range (100°K, 1000°K), χ p term may be reduced to the expression (6). These conditions lead to the expression (8) which is analogous to (1) providing that T 0 is replaced by δ E 2k . The numerical results of Ref. [5] show that e 2 a and 1 δE are roughly proportional to L . Therefore the magnetic susceptibility may be expressed in the form (7) where α 1 , α 2 and α 3 are parameters. The χ ' term represents the σ electron dimagnetism neglected in the London theory. The magnitude of this term is taken to be −0.3 × 10 −6 emu cgs. A value α 2 = 0.68 is obtained from Ref. [5]. The best agreement with the experimental results shown on Fig. 2 gives: α 1 = −0.056 × 10 −6 emu cgs α 3 = −3.5 × 10 4 . When each carbon is characterized by the parameter L , the experimental points are fitted well by the formula (8) (curves in full lines on Fig. 2). It can be seen that the L values are analogous to the experimental crystallite diameter L a . When the size of these samples is greater than 100 Å the agreement with experiments deteriorates (Fig. 6b) because of the interactions between graphitic planes which involve non linear variations between χ d , e 2 a and 1 δE with respect to L . The analogy between the expression (8) and (1) obtained respectively with continuum and discrete levels can be related to the analogy of magnetic susceptibility curves as the number of aromatic rings increases until the graphite is reached.