Many applications require the privacy of the sampled data while they travel from the source sensor nodes to data collecting device, say data sink. Providing an efficient data aggregation scheme with preserving data privacy is a challenging problem in the research of wireless sensor networks (WSNs). Although the secure data aggregation in WSNs has been well studied in the recent years, there exists a little work, for instance PDA (Privacy-preserving Data Aggregation), which focuses on protecting sensor data not only from adversaries but also from the participating trusted sensor nodes. However, PDA suffers from one main problem which is the high communication cost due to unnecessary traffics in the network during data transmissions. To resolve the problem, we, in this paper, propose a secure aggregation scheme of private data for WSNs. The proposed scheme applies the additive property of complex number to unite sensor data and for their privacy during their transmissions to a sink node. With our analytical performance evaluations, we show that our scheme is more efficient than the PDA scheme in terms of communicational cost.