Temporal resolution and crosstalk of an ultra-high-speed image sensor operating at 1 Gfps are analyzed by using Monte Carlo simulation. An impulse response method is proposed to analyze crosstalk and to cancel large crosstalk involved in output signals of the sensor. The sensor is backside-illuminated (BSI) with multi-collection-gate (MCG) at central area of a pixel on the front side. Signal electrons generated by incident light near the back side travel to the central area of the pixel and are collected by the collection gates. The travel time and the path of a signal electron randomly distribute due to randomness in arriving spot on the backside of the sensor and penetration depth of an incident photon which generates the electron, and in the motion of the generated electron. The travel time is analyzed to define the temporal resolution of the sensor. A collecting gate is defined as one of the collection gates which is supposed to collect all the signal electrons at a certain time interval. However, due to the randomness, some of signal electrons migrate to other collection gates than the collecting gate, resulting in mixture of signal electrons at neighboring collection gates and/or pixels, causing crosstalk in time and space. The proposed impulse method is used to analyze crosstalk when the sensor operating at 1 Gfps. From the analysis, a cancellation method of crosstalk involved in output signals is presented and successfully applied in cancellation of crosstalk in an example of 2 dimensional images.