Abstracted from the nested structure of biological cells with application on the modeling of economical processes, numerical P systems (in short, NP systems) as a kind of distributed parallel computation systems have been proposed. It has been proven that NP systems and variants are Turing universal for number accepting/generating devices and language generating device. However, universality of NP systems as function computing devices has not been established. Aiming at numerical P systems with thresholds (in short, TNP systems), small universality for computing functions is discussed in this paper. Six small universal function computing devices of TNP systems for two threshold cases and working on three different modes are constructed, respectively.