Wireless Sensor Networks (WSNs) has been utilized in a wide range of smart grids applications due to their capabilities to monitor environmental phenomena or connect the physical world to the virtual word. For smart grid applications, supporting the Quality of Service (QoS) is the main requirement from WSNs, such as transmitting delay-critical data from smart grid assets as fast as possible or identifying priority packets which need to be transmitted before any other periodic packets and hence reducing the collision rate. At the same time, WSNs need to provide the required QoS for a long time using the limited resources of the network. Meeting such a goal requires a remarkable design for WSNs protocols in order to satisfy the requirements of delay/priority critical smart grid applications. In this paper, we propose a suit of novel WSNs Medium Access Control (MAC) protocol which aims to provide QoS in terms of low transmitting delay for packets and supporting the priority packets over the network for smart grid applications. Our proposed MAC protocol is called Delay and Priority MAC protocol (or DPMAC) for smart grid applications based on WSNs. DPMAC protocol is based on the delay estimation and priority packets that are defined by the application layer of the smart grid application and the network conditions. Our comprehensive performance analysis shows how the proposed DPMAC protocol can achieve low end-to-end delay and lower collision rate in comparison with the well-known IEEE 802.15.4 MAC protocol which is used extensively in literature to provide the QoS for smart grid applications.