Preform, made of plastic material for the manufacturing of hollow bodies through a blow-moulding process, in which said preform comprises a substantially cylindrical body (1) having an open end (2) and a closed end {4), in which said closed end (4) has an outer surface with a varying curvature which is entirely enveloped within a hemispherical surface having a radius equal to the outer radius (R) of the cylindrical body (1) of the preform, said outer surface with varying curvature being tangent to the hemispherical surface in which it is enveloped in correspondence of the circular crown C and of the preform vertex.